Same way. Transfer money in once you're on the website and is not being blocked at bank level. They also take crypto so it's not like the bank can block it either even if France tried to block that.

BTW, I think the site and premis behind Polymarket is shit and nobody should use it, but blocking websites effectively has proven difficult at a technical level. (See PirateBay, still going strong despite everything entire governments and massive lobbying power can throw at it, and 4chan's refusal to be blocked by the UK's OSA) Most governments settle for the political win and blocking people with only a casual interest, which is fair enough. Most serious attempts at blocking specific sites tends just to make lawyers richer.