I've added both my kids to my finans account. They both had to get tax numbers. One did the forms when over, the other did the forms another time, sadly by that time they'd lost the form from the first one!
Managed to redo while all together in Antalya and the forms were couriered over to Fethiye.
Only problem is if I decide to change to a different product we would all have to be present to sign, but for rolling long duration savings accounts they don't need to sign each time.
I did it to save any problems with accounts being frozen should the worst happen.