Use an emote at the Stark Workshop
First of all, it is important to note that you must imperatively be equipped with the Tony Stark, which unlocks at level 93 of the Battle Pass, challenges them, starting at level 100.
The hardest thing here will be finding the Stark Workshop, since it's not explicitly marked on the map, and most importantly, it's only been available for a very short time. The latter is located more or less in the center of the map, right to the east of the Authority. It is a wooden shed, easily spotted. For the challenge to be validated, you just have to enter it and use an emote, which will automatically be in armor. Once done, you will unlock the Iron Man outfit.
We've given you the location of the Stark Workshop in more detail on our map below.

Le skin Tony Stark must be worn for the challenge to be validated.