It looks like you have it set up to send this to QuickBase as you have what QuickBase expects to see in your message body.
I would take a look at the Discord help documentation on webhooks and see what Discord expects instead. One word of caution is that Discord prefers JSON, not XML.
As far as the message header I think you would use "Content-Type" and then "application/json" respectively. You are going to want to make sure you are sending JSON though but you could try XML as well if you change the message header.
https://support.discordapp.com/hc/en-us/articles/228383668-Intro-to-Webhookshttps://discordapp.com/developers/docs/resources/webhook