You can mark a thread as SOLVED but you can't directly mark the specific post that gave the solution.
However, what would be welcomed by any forum member is to use the methods discussed in this sticky thread: https://www.access-programmers.co.uk/forums/showthread.php?t=296747
Basically, use the Thanks button and / or reputation points
As a new member you may not be able to allocate any RPs yet - these increase according to your own RP status & how long you've been a member