Hi Andy, there is a description on where to click in the help file here:
https://moi3d.com/4.0/docs/moi_command_reference8.htm#flow

For your case here you would want to click not directly in a corner, you need to click slightly away from the corner and then do the same on a matching location on the target surface.

So with a base surface click here:


You would then click on the target surface here:


- Michael