@Anonymous - I am sorry as the details were not clear from earlier description.
In Http Post Action, URL is something that you need to provide depending upon the service that you are calling. You should have that URL already. If that URL is from Another Flow which has HTTP Trigger, you will simply copy from that flow and paste here. If that URL is something else like an REST End Point OR OData Service end point given by SharePoint, you will use that one.
Http Trigger Autogenerates URL because caller needs to use it to call. Http Action expects URL that one has to provide so that action can call it.
Like and Mark this as answer if it resolves your issue.