The "tab to start on" is stored in a session variable.
p_web.setsessionvalue('showtab_MailboxesFormControl',0)
Tabs are numbered from left to right, with the leftmost one being Tab number 0.
You can set this at the top of the GenerateForm routine. So in your case a URL something like;
U_Membri?MEM__Nr=778&Change_btn=Change&tab=2
And code like this;
if p_web.IfExistsValue('tab')
p_web.SSV('showtab_procedurename',p_web.GetValue('tab'))
end
Cheers
Bruce