You are in the right place.
I'm speaking from memory here, I only have a tablet to hand and I've never managed to work out the editor under Android so I only use the Windows version to edit. Also, I don't use this method, I send all the OSC commands in script.
The simplest way is, I think...
Delete everything under Address and Arguments.
Click the + next to Address
Select Constant and add the whole command, as in Edward's example, as a string.
Thing is, TouchOSC (both versions), have a lot of idiosyncrasies that are going to drive you up the wall. MK1 is simpler and for, testing out the desk side, a great deal less frustrating. However it's simplicity means you can't build a nice looking interface or do anything subtle.
The editor in the Android version of MK2 is, frankly, awful - hence my recommendation you use the Windows version.
I did look at offerings from elsewhere but didn't find anything better for what I had in mind.
You think this is bad? Wait until you try scripts and the Lua language. I got through it but it was a struggle and my code still isn't pretty. It can be done 😊