This is such a universal need that there should be a simple universal hand sign which would immediately indicate such a need. Here is a suggestion for just such a sign.
The left hand is held palm facing downwards and the fingers together. The index and second finger of the right hand, held close together, are then placed across the knuckles of the left hand.

There is no symbolism in this sign. It is just a sign designed to be discreet but very clear.