Sorry, I've been working the last couple days so I haven't been on the forums.
Pretty much any command line script like "/tell player hello" or "/show_window MainHUD.Browser" can be used but without the "/" character.
Some scripts will just work like
<Button OnPress="tell player hello"/>
Some scripts don't seem to work and require you to write
<Button OnPress="show_window=MainHUD.Browser"/>
This is the "= style notation."
If you watch closely in the UIBuilder the very first time you press a button with the "= style notation" the press will just create a new property within that button so it looks like
<Button OnPress="show_window=MainHUD.Browser" show_window="MainHUD.Browser"/>
and the button doesn't actually do anything in the game until you press it a second time.
To work around this complication
UI modders started to just put the scripts in twice like
<Button OnPress="show_window=MainHUD.Browser show_window=MainHUD.Browser"/>
so the first script creates the property and the second script fires off the command.
If you don't want to put the script in twice you can preemptively put the property into the button so it's already there like
<Button OnPress="show_window=MainHUD.Browser" show_window="MainHUD.Browser"/>
so then the button will work the first time you press it.
I had to do this with my pet window so the button wouldn't spam your chat window twice.
<Button Name="PetMelee" OnActivate="pet=melee" pet="melee"/>
So, to summarize, some scripts just work and that's the first and easiest thing to try. Some require the "= style notation" which will require you to put the script in twice or preemptively add the property into the button.