I didn't figure this out personally, but adapted a posting in the forum of another file manager to Salamander.
To display the Windows "Run with" dialog for a selected file by clicking on a user menu item...
Build a new user menu item and configure it like this:
Command:
$(WinDir)\System32\Rundll32.exe shell32.dll,OpenAs_RunDLL
Arguments:
$(FullName)
Initial Directory:
$(WinDir)\system32\
Note: I think any path will work here
Execute through shell: YES
Open shell window: NO
Show item in User Menu Bar: YES