Actually you cannot use your present Mullvad account to do so. Instead, Mullvad provided exclusive API for tailscale as partner account to do so, which you can only get from Tailsacle if you choose their service. Mullvad only allows those data to go through its server.

Given that Tailscale is a ridiculous company that advertises on privacy while force user to login via SSO by tech giants or OIDC which is shit in privacy, I will stick to my current origin Mullvad account and keep away from this service.

In the most literal sense, they are still a "send me cash in a snail-mail envelope and we'll let you in" provider, see https://mullvad.net/en/pricing

> Can I really pay with cash?

> You bet, and please! Stay anonymous all the way. Just put your cash and payment token (randomly generated on our website) in an envelope and send it to us. We accept the following currencies: EUR, USD, GBP, SEK, DKK, NOK, CHF, CAD, AUD, NZD.