When I choose nheko as a client, matrix.to will already offer links in the form:
matrix:r/lobby%3Asspaeth.de?action=join&via=sspaeth.de which will work in gomuks, EXA, and all other clients that implement a propose matrix: URL scheme.

This is the matrix URI scheme that has been in the spec since v1.2. It would be great if there were a client-independent and generic choice that I could make, to get client-independent links.

(Merge request to fix this upcoming, "Learn more" points to the matrix spec)