I only know how to do it manually by editing the url. Say you want to visit “asklemmy” on lemmy.ml and “asklemmy” on beehaw, but through lemmy.ml. The two URLs for this look like:
lemmy.ml/c/asklemmy lemmy.ml/c/asklemmy@beehaw.org
This may be a bad example because I don’t know if beehaw actually has an asklemmy community.
Personally I use memos as a Google Keep replacement. There are mobile apps for it as well called moememos.
Check out https://usememos.com