Tommy Cooper used a script very similar to this before performing a magic trick:

Tommy Cooper: Ladies and gentleman, this next trick that I’m about to perform for you is so dangerous that I may die during the trick. Therefore, just in case I do not make it to the end, I would like to take my applause…. now!

[audience applauds]

Tommy Cooper (sounding disappointed): Oh well for that, I won’t bother doing it then!

This script itself is very clever in that it doubles as both a joke and an applause generator. The audience will feel compelled to applaud as otherwise they feel they might not get to see the trick. If you use this line you could then perform a card trick or sponge balls trick that involves no real threat to your life as a bit of humor afterwards.