In addition of Daniel suggestion, and after doing that, I managed to assign slashes to your example’s last notes highlighting it singularly and, in the context menu (right click) go to “Percussion/Change Voice…” (and not to “Voices/Rhythmic Slashes…”) and choose the ones that begins with the “thousand” 1001 or 1002 or similar…
I hope this is the right way… but it works.
DanielMuzMurray I didn’t know the procedure was different for percussion. Thanks, will take a look at it later, but as of right now I’m trying to figure out why my bass drum is not playing. The noteheads look normal. I don’t know what I did that broke the playback.