Not sure - I actually selected the program from the drop-down list, and then when I input just my TK account number, it did not accept (saying that it is not correct). After putting in the 'TK' prefix, it did accept. However, before initiating my first transfer, I did some research and in fact found a few posts in FT itself suggesting one may need to use 'TK' prefix before the number.

My suggestion: if Marriott doesn't accept your membership number, try adding the airline prefix and see if it is accepted. Also, for the first try, I would try to transfer the least amount of points allowed.