Like Halcon said, just try scanning in with boarding pass. If manual check-in is required and you get push back, I find it helpful in these situations to understand what rule the agent is getting tripped up by. In this case, explain that Delta removed the benefit for Medallions to get lounge access when flying Y, but the benefit remains for other airline Skyteam Elite Plus as DL has a contractual agreement to Skyteam for that benefit. A good agent will hear that and re-read their rulebook in KM more closely and will likely understand the nuance of the rule wording once they actually know what they are looking for.