When you check in online, there is only a drop down list of requests. I have never found what I wanted to request in the drop down list, and there is no place to add in a request by typing.
I do what disneynutz does and e-mail MS using the link on the Member web site.
For what it's worth, I've had pretty good luck with requests done this way. I think it helps that I keep them very general...ie, upper floor, near elevator, etc. Never room numbers or even a block of room numbers.
So far, I haven't seen any advantage to online check-in for us members. I guess if there's a big line at the front desk it would be helpful but when we've used it (just twice) we didn't see any improvement in check in speed, room assignment as regards requests, or room availability sooner than just checking in at the front desk.