I can't help much as I'm currently discovering that code. But, have a look in hr_timesheet_sheet module on the first tab of timesheet there is a widget showing a m2o on account.analytic.account.

This is the module I used to inspirate myself. I finally found a way to do what I want by using javascript to create the many2one field. Anyway, thanks a lot for your message.

Hello Simon how you did it?