The problem was that I couldn't figure out first how to use custom HVars, as the commands I needed were missing from the events.txt file, maintained at Mobiflight at the link you mentioned above.
Once I had learnt how to use Hvars in FSUIPC and followed your advice about creating a custom myevents.txt file in the main FSUIPC folder, with the custom HVars that I wanted to use (found in the \Community\hpg-airbus-h145\html_ui\HPGH145-System\H145_Keys.txt file), I was able to assign these commands to keybinds in FSUIPC, with MSFS active and the H145 loaded.
My mistake was to try and use a trim release command in a helicopter with an advanced AFCS (auto-flight control system) that I didn't completely understand.
Once I had assigned the commands correctly on press and release, and was in the air with AFCS engaged, I was able to correctly release the cyclic trim, to allow for precise control when needed.
Thank you for your help John.