Is it possible to use my UNI Membership to participate in third-party competitions listed on UNI?

Membership entries apply to competitions hosted by UNI, not to listings run by outside organizers.

No. A membership entry can only be spent on a competition that takes its registrations on UNI, and competitions listed by outside organizers do not.

Hosted and listed competitions

A competition card showing the 'Hosted by' vs 'Listed by' label, since a previous incorrect screenshot of this was removed and never replaced.

UNI carries two kinds of competition, and every competition card tells you which one you are looking at:

Why membership does not apply to a listed competition

On a listed competition there is nothing on UNI to register through. Depending on the listing, the page either shows no Register or Submit button at all, or the button opens the organizer's own site in a new tab. Either way your registration, your fee and your submission are handled entirely by that organizer, on their terms and at their price, so a UNI Membership has nothing to attach to.

What your membership does cover

On competitions hosted by UNI:

Membership tiers and pricing lists the included entries for each tier.