I see videos where people take their shahada and the imam will tell them to repeat something like this: وَأَشْهَدُ أَنَّ عِيسَى ابْنَ مَرْيَمَ رَسُولُ اللَّهِ after the normal shahada, would this be considered a bidah since this wasnt done during the time of the Prophet?

Ustadh AbdulAziz al-Haqqan

It is allowed but not binding.