Can I encode an NFC tag to send HTTP POST data? Can I encode an NFC tag to send HTTP POST data? Or can I only use HTTP GET?

I have Googled extensively, but can't seem to find an answer (which, I suppose, implies that it is HTTP GET only, but I would like someone to say so).
No, you can only encode a URL on the tag. The tag neither contains an instruction to issue a HTTP GET request nor a HTTP POST request.

How the URL is interpreted and processed depends on the app that opens the URL. Typically that app will be a web browser and that web browser will typically treat the URL as if it was typed into the address bar. Consequently, the web browsers usually opens the URL by issuing a GET request.