If they could be using web UI (I'm not sure how a client would work with auth like that) then it kind of depends on how much hassle they can live with and how secure you want to be

Simplest would be to use https://doc.traefik.io/traefik/reference/routing-configuration/http/middlewares/basicauth/
You set up username and password and share those with the user. But it can be brute-forced

Something more secure but also a bit more demanding would be some kind of email otp
https://docs.goauthentik.io/add-secure-apps/flows-stages/stages/authenticator_email/
Set up authentik to send time-limited link to their email that well let them through. But they would have to authenticate every time they access