UPDATE and FIX
As @Michael_Taggart mentioned above, the following should work:
- Present your prototype first. IMPORTANT!
- In the presentation window, at the top right you’ll see “Share Prototype”. Click that
- Then click the “Copy link” at the top of the small modal
If you click the “Share” button in Figma while designing and choose that “Copy link” it will not work. Apparently that’s sharing a link to edit the file, not view the prototypes. (even though both the “Share” and “Share Prototype” buttons bring up the same modal.