How to add extra client to multi-client studio playtest?

To debug some performance issues, I rely on the ability to add a new player to my studio playtest session while there’s already an ongoing session, for accurate profiling.

I start a multi-client playtest session

…But from here, there doesn’t seem to be any way to add another player?
I’ve searched through the tab customization options as well as studio settings, to no avail.

The reason I ask…

…is because a while back, Roblox briefly updated the studio UI with different layouts and looks (before presumably switching back to the old one due to it being a beta?).
It replaced the player amount dropdown next to “Start (F7)” with an option “Add Player”, which made a new client start up and had them join the ongoing playtest session.

I’m heavily reliant on this feature, as I can’t debug player join/leave issues without it.
It also specifically must be multi-client playtest (F7), due to issues surrounding the “play here” feature with the game I’m working on.

Is it hidden somewhere and have I just not looked hard enough, or is it truly just removed?
There’s also no mention of it in the docs.

Thanks in advance!

I’ve enabled the next-gen studio beta and was able to find the feature with ease.
I’m guessing this just means it’s not implemented in the old one?