DeeRSoos From what players are saying, the big thing is that the smoke itself needs to be helping your team get the takedown. So it is not just about throwing smoke out as Support, it is about placing it where teammates are actively fighting and getting kills through it or right around it.
What seems to work best is running Support with Smoke Grenades, staying stocked on ammo, and using your smoke on chokepoints, objective entrances, and busy push lanes in modes like Breakthrough or Rush. It also helps to keep your smokes going back-to-back instead of tossing one too early and letting it clear before the fight starts.
One thing to keep in mind, too, is that revives in smoke do not seem to count for this challenge. It is specifically tied to smoke assists, so the goal is to help create cover that leads to teammate kills while the smoke is still active.
I had a thing on this written up; if I can find it, I will send it to you.