A post similar to this exists, here.
As @Cinderingmentioned in that thread, console’s osk popups up if any TextBox is selected using the UI navigation controls.
I’m the same as you. No Xbox, but a third party controller that works with roblox.
However, my cousin has an Xbox and from what I’ve seen, roblox has their own Xbox keyboard that’s built into every game unless you tell it to use yours.
This info might be wrong since we didn’t play roblox that much, but I’m pretty sure I’m right.
Also, you can just make the text box with no keyboard. Then play roblox on your computer but connect a controller, roblox recognizes most controllers (my 8bitdo one worked but not joycons without extra coding in my game) and it should work if you click the text box. Make sure to try it in other games too though just to make sure.