Enter formula into ActiveCell

Not tested but try this:

Sub CreateNewVO()
'Creates the interactive dialog boxes, after inserting cells in a selected range.

Do
    Range("A3").End(xlDown).Offset(1, 1).Select
    
'A. Inserts ID number formula
    Value1 = "=IF(" & ActiveCell.Offset(0, 1).address & "="""", """", MAX($B$3:" & ActiveCell.address & ")+1)"
    ActiveCell.Formula = Value1
    ActiveCell.Offset(0, 1).Range("A1").Select
End Sub