Should one make up the Witr prayer upon feeling something white discharge during the prayer, when uncertain if it exited before or after the prayer?

If you verified that something you felt descending during prayer actually exited, then your prayer is invalid and your ablution is nullified. It was obligatory for you to exit the prayer when you felt that. You are not required to make up the night prayer, for supererogatory prayers are not obligatory to be made up. Indeed, most scholars hold that supererogatory acts of worship are not binding upon commencing them, and it is not obligatory to make them up if one exits them, unlike Hajj and Umrah (definition). Some scholars, like Ahmad, are of the view that prayer is more serious and should not be interrupted; if it is interrupted, it must be made up.