1. Create new field as picture below

2. Go to the form, right click and choose restore to reload new field we added before. On the field method, add code below
 

public Common lookupReference(FormReferenceControl _formReferenceControl)
{
    return NumberSequenceTable::lookupReference(_formReferenceControl);
}

3. Add reference group for the field at the form as picture below , and you are done.

4. Just run the form for testing.