How can I contact a player I had a match with?

If player you want to contact was playing in a match with you,you can go to your profile inside starcraft 2 and in your match history you can find and chat with all players from the selected game.

If you know someones character name you can go an try to add friend > character friend and type in his character name.(Not sure how it works when multiple payers share same name).

In your case if you are looking someone "popular" and active on forum you can go to starcraft 2 website and use his name in search bar,if you are lucky the results will show like this,where you can see his full character name along with the ID tag...Now you are back to trying to add him as a friend and trying to chat with him.