You should test you BO's editing state. As in:
If YourBO.EditingState = MicroFour.StrataFrame.Business.BusinessEditingState.Adding Then
Textbox1.Text = modEmployee.careerhis_sequence_increment(BO_hretCareerHis1.iChsEmployee_FK)
End If
Or you can use the BO's EditingStateChanged event and set your isnew property:
Private Sub YourBO_EditingStateChanged(ByVal e As MicroFour.StrataFrame.Business.EditingStateChangedEventArgs) _
Handles YourBO.EditingStateChanged
If e.EditingState = MicroFour.StrataFrame.Business.BusinessEditingState.Adding Then
isnew = True
Else
isnew = False
End If
End Sub