I think the original question was not really answered, and I happened to find the source of a document that answers the question.
// ‘Tight’ integration into Sonic Pi
Integrating your synth into Sonic Pi in a tighter manner will allow you to use it without having to enable external synths and FX or explicitly load the synth path into memory beforehand. It will also enable features such as synth opt autocompletion.
However, there’s a little more involved when doing it this way. You need to add the appropriate metadata to
app/server/ruby/lib/sonicpi/synths/synthinfo.rb and re-compile the app.
Lastly, as with built-in synths, you would call the synth by name with a symbol. Eg:
synth :piTest .
We look forward to hearing about your synth and FX creations - have fun!//
From: sonic-pi/SYNTH_DESIGN.md at main · sonic-pi-net/sonic-pi · GitHub