What is the ruling on praying a supererogatory prayer while sitting when feeling the urge to relieve oneself, especially when one is able to pray standing and does not wish to pray sitting in front of people?

It is permissible for you to perform supererogatory (nafl) prayers while sitting, even if you are able to stand, but your reward will be half the reward of one who prays standing. However, if you are unable to stand, you will receive the full reward of one who prays standing. As for prayer while resisting the two impurities (urine, feces, or gas), it is disliked (makruh) and diminishes the perfection of the prayer. If the resistance is very strong, the prayer becomes invalid according to the Maliki school. One should not be concerned with the dislike or shyness of praying while sitting in front of people, for Allah is more deserving of shyness (reverence) from us.