Hi Peachy, I had the exact same problem and couldn’t figure it out so just created a development wallet using Stuart’s script here: Development Wallet Creation
This will allow you to create the necessary hex encoded keys to play around with txs using the API.
I believe 0xOmar in Discord has also created a python library that can extract the private key from the desktop wallet seed, so you could look into that if you want to use the desktop wallet created address.