You have choices, short of finding the squeak yourself.

1- you can tell the shop they either find and fix the problem or take it back and refund your money.
2- you can ignore the squeaking and just live with it, but ONLY if the shop accepts responsibility for any consequences of ignoring the "problem".

3- you can become a detective, and identify the specific origin. You do this by process of elimination, by treating one item only, until you find the one that matters. That's how I suggest the shop approaches the issue, but it's up to them

Also keep in mind that it may have nothing to do with the suspension, and it might be the seatpost or saddle itself.

I always attack these by what I described method 3. To make it easier, I made up a bottle of heavy oil, thinned with naphtha until it pouts like gin. I apply it to suspect areas and let it wick in. I do one place at a time until I find the one that solves the problem. Because there's so little oil, the problem will come back, but now I know where to put my attention.

Also note that if there are any rubber parts involved, you may do better with a silicone grease (also sold as O-ring grease), than one intended foe bearings.

So, that's all I have for you, especially since I'm working deaf and blind, but maybe if you bring this to the shop, and speak to them about getting this solved in X number of tries or take the bike back, you can motivate them to get this solved for you.