Can Adguard AddOn use the letsencrypt certificate?

Is it possible to use the certificate created by letsencrypt, and if so, what do I enter in the path for the certificate and key?

Yes,

They should be mounted and can be found at
/ssl/fullchain.pem
and
/ssl/privkey.pem

Hope it helps.

Ahh, so simple, works perfectly, thankyou!