You could put it in a bash script and put the script on your desktop or possibly drag that script to your dock (depending on the flavor of Ubuntu), something like
#!/bin/bash
cd /home/(username)/Downloads/tiddlywikilocations/ (or wherever you have it)
./nw
or you could use the keyboard launcher Kupfer and do this:
copy the nw file in a file manager, make sure that its executeable
launch Kupfer and paste.
tab until it says "run/execute" but DON'T enter --> instead do control-enter, then tab, then "T" to add trigger (a shortcut) and then it will ask you to do a keyboard shortcut combination to launch tiddlydesktop whenever you want (although Kupfer has to be set to run on boot so its always on)
(this is from memory as I'm currently upgrading my Ubuntu to 20.04 from 19.10 and things aren't working right now :)