Hi @Anonymous,
Like suggested in my previous reply. You could list the existing hub sites and dynamically pick one of those.
Below is an example of that approach. In that example I am retrieving all the hub sites (via the Send an HTTP request to SharePoint action).
In the Join hub site I am retrieving the ID of the second hub site that is listed in the response of that request via the following expression.
outputs('Send_an_HTTP_request_to_SharePoint')?['body']['d']['results'][1]['ID']
