I would like to reserve the 7th slot, if I am eligible.
I think UniJoin meant something else. Specify the slot number along with the text you would write in this thread, and not just “specify the slot number” because in this case you would be better off sending your preference to them in a private message.
Unijoin will confirm, but I think Etranger is doing right:
--snip-- Therefore, next time you discuss in this thread, please indicate the slot number that we should reserve for you (or a random one). Or write to us in a PM indicating the slot number.