If you select a spot on an edge, it'll place the mark there. Then if you drag it away, it'll grow an extension line.
Works for me, anyway (SW2016).
I can select a surface and drag the finish mark away and it works for me as well. I have learned something I did not know today! When I checked settings, the drafting standard is set to ANSI, and it clearly had the leader option checked, but I did not realize that one had to drag the finish mark away to create it. Thanks!