For Organteq, I suggest you take a look at some earlier threads that discuss its use with Dorico, e.g.:
To use another virtual instrument instead of the HALion Sonic SE piano, you should create a new playback template that combines the existing HSSE+HSO (Pro) playback template with your new chosen piano instrument or sound. You can read about how to create playback templates here:
You may also find this video helpful:
And here is a more in-depth exploration of creating your own playback template:
