How to create a customized Zapier integration with Xero

I’m trying to set up a personalized integration on Zapier that enables me to generate quotes within Xero, which isn’t a built-in feature. I’ve been referencing various resources to guide me through this process. Currently, I am in the phase of constructing the action and running tests with sample data. Unfortunately, I received a 400 error when attempting to POST to Xero’s token endpoint, indicating a likely issue with how I’m formatting the JSON data. I’ve made attempts using both ‘formatted’ and ‘raw’ methods, but I’m still encountering problems. I would appreciate any insights from someone experienced with Zapier who has successfully developed their own integration with Xero.

EDIT

I’ve redacted specific IDs for privacy, but noted that I missed concealing the contactID in an earlier screenshot.