In reactive gateway, I can do:

spring:
  cloud:
    gateway:
      routes:
       - id: cashcards
          uri: http://localhost:8090
          predicates:
            - Path=/cashcards/**
          filters:
            - TokenRelay=cashcard-client

In this example, I'm specifying cashcard-client as the registrationId to use for performing a token relay, instead of relying on the default behavior which resolves the registrationId from the current user (via the principal, which is an instance of OAuth2AuthenticationToken).

Note: The same can be accomplished with the DSL using f.tokenRelay("cashcard-client").

The same support and overloaded DSL option could be added to TokenRelayFilterFunctions for the mvc gateway. I'm happy to raise a PR for this.