Is it permissible to make supplication while sitting after the salutation following the eighth rakʿah in Qiyam al-Layl?

It has not been authentically reported from the Prophet, peace and blessings be upon him, that he would separate the rak'ahs of aqim al-layl (night prayers) and Witr with a supplication. It is better for the supplication to be during prostration, and after the tashahhud before the taslim of the prayer, or after completing all of the night prayers, especially in the last part of the night, as it is among the most hopeful times for supplications to be answered.