So instead of entering a receiving address and an amount one would enter '{"data":40_bytes}' ?

Yes, basically it comes down to this.

Here is an example:

1. First, I list unspent outputs of the address I'm going to use.

Code:

bitcoin-cli listunspent 0 999999 '["1BxtgEa8UcrMzVZaW32zVyJh4Sg4KGFzxA"]'

I have a problem trying to decode the data back
Is there any way I can see the data I added while creating raw transaction without using this website? --> https://live.blockcypher.com/btc/tx/b394e47f8141f725f25854d57e91fb67d91782dbe581b2f449990ba25a116d2d/