Well, "start" is their word for launch. And the launch pad, for that matter.

Rollout of Soyuz at Vostochny Cosmodrome and - no buckets!

https://www.youtube.com/watch?v=VdUc7i_NUkw

Like Kourou, the vehicle stays vertical on the pad at Vostochny longer than at Baikonur. (They roll a vertical shelter over the vehicle on the pad at Vostochny). I definitely think there is some sort of fluid which is added when horizontal at Baikonur, but is likely added while vertical at Kourou for Vostochny - so, no need for "catch buckets" during those rolls.

Here's a couple older pics (2008?) one showing the booster vertical with the cans still in place. The hoses look like they run up to some fitting on the side. The other shot shows, at least on the core, the can has been removed. Dessicant? Hydraulic drain or make-up?

I still think the latter two. If it was desiccant, you would for sure want that in Kourou!