Just type the letters: thisisunsafe.
Just click anywhere on the tab to give it focus (no button), and type the letters. (The letters will not be visible or show up in a field or anything.) After completing the words, the website loads, but will show as ‘insecure’ in the left of the address bar.
Edge will mark the website as “allowed”, unless this operation is done in an inPrivate window. After it’s saved, it works even with inPrivate.