Both of these are legal, but they might or might not mean something different depending on how you look at it.

Generally “Whenever X, Y” means that X happened/happens/is supposed to happen many times and each and every time Y also happens (or in this case, you’re told to make it happen). So your sentence would read as a general rule – “you’ll have many opportunities to hold onto happiness in your life, and each time that happens, make sure to do so”.

What is meant by “when X, Y” can be a single state! X happened, and Y happened at the same time. So you’d rather say “…when you get the chance” if there’s a certain chance – for example, you know your mate is getting a new job, and he’s being anxious for it, so you’re telling him to hold onto that once he gets the opportunity (and that job) the person got. What a chance (obviously) that you think he’s not ready to give up? Compare “When you get that job, you’ll be out of trouble” vs. (technically grammatical, but making no sense).! “Whenever you get that job, you’ll be out of trouble”

Now what what @Man_From_India said also holds – if the context or grammar already estabilishes that you’re talking about a repeating occurence, you can use “when” in place of “whenever”. What should your locksmith locks look like when you leave your own house?