AWS Websocket route to lambda by ARN

I can see in the aws api gateway console that I can choose any existing lambda for a custom websocket route.
How can I configure my sls yaml to route to a lambda that is defined in another project? All the examples show a handler that is local to the current serverless.yaml:

    handler: handler.fooHandler
      - websocket:
          route: foo # will trigger if $request.body.action === "foo"

What I need to do is point this handler to a lambda using its ARN