on AOT dataset method init() :

X++ code on init method example:

public void init()
{
    super();
    DNLLeaveRequest.DNLEmplRecId  = DNLEmployeeTable::findByUserId().RecId;
    DNLLeaveRequest.LeaveDateFrom = today();
    DNLLeaveRequest.LeaveDateTo   = today();
}

or you can code it on Tables\initvalue()

XML code on visual studio for EP pages :

<%@ Control Language="C#" AutoEventWireup="true" CodeFile="TIDDNLLeaveRequestEP.ascx.cs" Inherits="TIDDNLLeaveRequestEP" %>
<dynamics:AxDataSource ID="AxDataSource1" runat="server" 
    DataSetName="TIDLeaveRequestDS" ProviderView="DNLLeaveRequest" 
    oninitializeddatasetrun="AxDataSource1_InitializedDataSetRun">
</dynamics:AxDataSource>
<dynamics:AxForm ID="AxForm1" runat="server" DataSourceID="AxDataSource1" 
    DataKeyNames="RecId,Added,AdvanceUsed,BlankLeave,carryused,ContactPhone,createdBy,createdDateTime,dataAreaId,DNLEmplRecId,DNLLeaveTypeRecId,IdApproval,IdPersonSubstitute,LeaveDateFrom,LeaveDateTo,modifiedBy,modifiedDateTime,Remarks,RequestDate,State,StatusFlag,SubmittedBy,SubmittedDateTime,TableId,TotalLeaveDays,TravellingLocation,getEmplName**,GetEntitlement**,getExpiration**,getLeaveDesc**,mostRecentComment**,requiredActionDueDate**,titleFields**" 
    DataMember="DNLLeaveRequest_Current" AutoGenerateInsertButton="True" 
    DefaultMode="Insert" UpdateOnPostBack="true" oninit="AxForm1_Init"  >
    <dynamics:AxMultiColumn ID="AxMultiColumn1" runat="server">
    
 
    
 
        <dynamics:AxColumn ID="AxColumn2" runat="server">
            <dynamics:AxGroup ID="groupLeaveRequest" runat="server" FormID="AxForm1" 
                Caption="Leave Request" CaptionAlign="Top" HorizontalAlign="Left" 
                Width="300px">
                <Fields>
                    <dynamics:AxReferenceBoundField DataField="DNLEmplRecId" 
                        DataSet="TIDLeaveRequestDS" DataSetView="DNLLeaveRequest" 
                        SortExpression="DNLEmplRecId" Mandatory="True">
                    </dynamics:AxReferenceBoundField>
                    <dynamics:AxReferenceBoundField AutoPostBack="True" 
                        DataField="DNLLeaveTypeRecId" DataSet="TIDLeaveRequestDS" 
                        DataSetView="DNLLeaveRequest" SortExpression="DNLLeaveTypeRecId" 
                        Mandatory="True">
                    </dynamics:AxReferenceBoundField>
                    <dynamics:AxBoundField AutoPostBack="True" DataField="LeaveDateFrom" 
                        DataSet="TIDLeaveRequestDS" 
                        DataSetView="DNLLeaveRequest" SortExpression="LeaveDateFrom">
                    </dynamics:AxBoundField>
                    <dynamics:AxBoundField AutoPostBack="True" DataField="LeaveDateTo" 
                        DataSet="TIDLeaveRequestDS" 
                        DataSetView="DNLLeaveRequest" SortExpression="LeaveDateTo" 
                        Mandatory="True" >
                    </dynamics:AxBoundField>
                    <dynamics:AxBoundField AutoPostBack="True" DataField="Remarks" 
                        DataSet="TIDLeaveRequestDS" DataSetView="DNLLeaveRequest" 
                        SortExpression="Remarks">
                    </dynamics:AxBoundField>
                    <dynamics:AxBoundField AutoPostBack="True" DataField="TotalLeaveDays" 
                        DataSet="TIDLeaveRequestDS" DataSetView="DNLLeaveRequest" 
                        SortExpression="TotalLeaveDays">
                    </dynamics:AxBoundField>
                    <dynamics:AxBoundField DataField="GetEntitlement**" DataSet="TIDLeaveRequestDS" 
                        DataSetView="DNLLeaveRequest" ReadOnly="True">
                    </dynamics:AxBoundField>
                </Fields>
            </dynamics:AxGroup>
        </dynamics:AxColumn>
    </dynamics:AxMultiColumn>
</dynamics:AxForm>

C# code on visual studio for EP pages

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.DataVisualization.Charting;
using System.Web.UI.WebControls;
using Microsoft.Dynamics.Framework.Portal.UI.WebControls;
using Microsoft.Dynamics.Framework.Portal.UI.WebControls.WebParts;
using Microsoft.Dynamics.AX.Framework.Portal.Data;
using Microsoft.Dynamics.AX.Framework.Services.Client;
using Microsoft.Dynamics.Framework.BusinessConnector.Session;
using Microsoft.Dynamics.Framework.BusinessConnector.Adapter;
using Microsoft.Dynamics.Framework.Portal;
using Microsoft.Dynamics.Framework.BusinessConnector.Proxy;

public partial class TIDDNLLeaveRequestEP : System.Web.UI.UserControl
{
    DateTime dateFrom;
    DateTime dateTo;
    Int64 recIdLeaveType;
   
    

    #region customMethod
    private ISession AxSession
    {
        get 
        {
            AxBaseWebPart webPart = AxBaseWebPart.GetWebpart(this);
            return webPart == null ? null : webPart.Session;
        }
    }
    static AxBoundField GetField(DataControlFieldCollection fields, string name)
    {
        foreach (DataControlField field in fields)
        {
            // Is this the field being searched for?
            AxBoundField boundField = field as AxBoundField;
            if (boundField != null && String.Compare(boundField.DataField, name, true) == 0)
            {
                return boundField;
            }

        }
        // Nothing found, so return null.
        return null;
    }

    private DataSetViewRow CurrentRow
    {
        get
        {
            try
            {
                DataSetView dataSetView;
                dataSetView = this.AxDataSource1.GetDataSet().DataSetViews["DNLLeaveRequest"];
                return (dataSetView == null) ? null : dataSetView.GetCurrent();
            }
            catch (System.Exception)
            {
                return null;
            }
        }
    }

    #endregion customMethod

    protected void AxForm1_Init(object sender, EventArgs e)
    {
        //this.loadFirst();
        this.AxForm1.DefaultMode = DetailsViewMode.Insert;
        this.AxForm1.AutoGenerateInsertButton = true;

        AxBoundField axbfNoNPK;
        axbfNoNPK = GetField(groupLeaveRequest.Fields, "DNLEmplRecId");
        axbfNoNPK.ReadOnly = true; 
    }


    protected void Page_Load(object sender, EventArgs e)
    {

    }
 
   
    protected void AxDataSource1_InitializedDataSetRun(object sender, AxDataSourceDataSetEventArgs e)
    {
        e.DataSet.Init();
    }
}

preview :