As far as I know, there are a few different link formats, and how well they work depends on which frontend you're using:
- amateur_radio
- !amateur_radio@lemmy.radio (The Web interface gave me autocomplete options for this)
- https://lemmy.radio/c/amateur_radio
EDIT: At least using the web app, the first link is relative, and the others are not. So I think the correct format would be /c/<community>@<instance> for communities outside your instance.