Ok I have found that I can embed in the "GenerateForm" embed point and get it to open a specific tab. My problem is now how to pass the tab number from the calling procedure.
I have tried setting a session variable "StartTab" when the button is pressed that launches the form, but I cannot find an embed point that is triggered when a button on a form is pressed? It seems that if you set the "On Click" to run a procedure, then the Server Side code is never run.
I could perhaps embed the "On click" called procedure in the Server Side code, but not sure what to embed - I am calling procedure UpdateUser_Self with a parameter of 'change_btn=change&_bidv_=' & p_web.AddBrowseValue('pageheadertag','Users',USE:ById,p_web.GetSessionValue('LoginUserID'))
... so that the form is opened from another form at the correct record.