Hi,
Base on your description, you want to set the form's focus on the textbox1, right?
You can try to do like this: Me.SetFocus(Me.TextBox1), this code can works on the page with master page on my machine.
The SetFocus method's param is a control object, but Me.TextBox1.ClientID is a string.
Hope it helps.
Please remember to mark the replies as answers if they help and unmark them if they provide no help.
Yours sincerely,
Amanda Wang
Microsoft Online Community Support