Yes you have to be on that persons wishlist page. Make sure you have all the particulars correct firs ie ISBN, cover type (hard cover, paper back etc) author, title before you click post this book. If you even click on the description and try to post from there it will go directly to the first in first out order. Also make sure the persons account is not on hold. If it is and they do not have their books auto requested then it will go to FIFO anyway.

Yes, please make sure the ISBNs match! I had someone post a book directly to my WL once, thinking they were doing me a favor, and it was the wrong book...I had the HC version listed and they posted and sent it...and it was the paperback version. This was a book I already read and was looking for the HC to add to my "keeper" shelf, so I ended up having to mark it "RWP" for being the wrong book. (The ISBNs were totally different!)

And then make sure as Chris said, that you are posting it directly from that person's own Wishlist page so it doesn't go to FIFO.

Cheryl

I usually ask first incase they're out of credits or already have the book and forgot to delete it from their WL. 

Also if your account is on hold-take it off before posting the book or it'll go to FIFO.  I learned this the hard way.  I have my account on hold because I'm moving cross country and can't have any book sent to me right now.  But I can still send out books.  I was posted a WL book to FIFO.  I checked that person's WL and I had another book I could post to them.  They wanted the 2nd book but I forgot to take my account off hold when I posted it and it went to FIFO as soon and I took it off hold.