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.