Should I use a 240V extension cord?

Yes, but be careful. A standard 120V extension cord will fry itself on 240V, causing a fire. For a window air conditioner or welder, you need a cord rated for 240V and the correct amperage (e.g., 10 AWG for 30A) with the right plug (NEMA 6-15 or 6-20). Avoid them if possible—window ACs should use a dedicated outlet, and welders must follow the manual’s cord specs. If you’re asking, you probably shouldn’t.