Add mount to hotbar

[Bun_Vivant]

Is this the only way, I used to be able to select and add to hotbar through the mount guide. I've been playing on steam deck. If the only way is to drag and drop, I guess I will have to boot up the game on my PC and do it.

I don't know Steam, but hotbar assignments are local to the PC so it's not clear to me that changing them on the PC will fix your problem on Steam. Can't hurt ti try, I guess.
Have you tried writing a macro to summon the mount and assigning that to your hotbar? I do that to have macros that automatically level-sync me when I dismount, for when i'm running FATEs.