I’ll preface this by saying I’ll use any app that can achieve this result, but I’m using nfc tools right now.
I’m trying to program my xNT to launch an application (easy) and then automatically add predetermined text into a message field (impossible?).
Going full geek, form over function, with this.
This is going to be hard to do without root, apps don’t like other apps injecting text into them 
Yeah I agree. It depends on the app though as there may be other ways to accomplish it (like open a special URL that puts stuff in the app). What app are you using?
This is also true, you might be able to (ab)use android accessibility permissions also.