Even if you wanted to withhold rent, unless your rent is literally due tomorrow, or in the next couple of days, it's not going to matter. You have to allow a landlord a reasonable amount of time to diagnose and fix an issue before you can withhold rent, and it sounds like they're already working on it. This weather has caused issues for a shitload of people, and I'm sure the repair guys are swamped, right now, so most people aren't going to be able to get things fixed the same day.

Instead of trying to learn how to threaten to withhold rent, why don't you try asking your landlord to get you some space heaters since the radiators aren't working? You've already admitted that they've acknowledged the issue, and are working to get it repaired.