I'm rather new myself, but if I'm correct you don't need to apply to be a middle man.
More than likely, however, you'll have to gain a LOT of trust to be a decent middle man, and probably have to offer free services for awhile before you start charging. Middle men are supposed to be some of the most trustworthy people on sythe - it'll take some time
If you mean OMM (Official Middle Man) you'll have to do a LOT of trades with quite a few over $100, if I am correct.
Hope I helped!
Edit: Apologize for posting here when you asked for MSN, but too lazy to add ya