I was thinking about something like: " I will laugh" but it seems kinda unnatural
I guess nobody could post an answer for sure until further context provided, but the situation I first came up with looking at the phrase 笑っておく is "do something superficially or for manners' sake", as in (loose translation):
I didn't understand any English joke, but people around me were laughing, and so did I.
My boss made a lame joke, but I decide I'm going to laugh to save his face.
In the case of 笑っておく, the third definition is the most likely one. You can probably translate this as:
- to laugh at something for now (not knowing how else one can respond to someone's statement)
- to laugh at something anyway (and forget it)
- to just laugh at something (and let it lie)
As @Flaw said, "to laugh beforehand / in preparation" is a less likely but possible interpretation, depending on the context.
Depending on the context, I think it may have the sense of:
- "I better laugh now, in case circumstances change such that it becomes inappropriate or difficult to laugh later"
- "I expect what's about to happen to be funny, so I'll save you the time and I'll mockingly laugh ahead of what you're about to do"
Of course none of these are actual translations, i.e. you wouldn't replace 笑っておく in a conversation with the above sentences.