Hi @Michael_Leahy,
You are going to be one of the very few developers that uses Delphi to develop Keap API Integrations. Most developers will be using PHP, Python, NodeJS, .NET, plus a few other languages.
There is a Facebook Keap API Community which you can also post your question in, which may have another Delphi developer on there, but no guarantees.
If that does not help, you do have other alternatives to connect to the API if you want to try things out. The Legacy API Encrypted Key and OAuth Personal Access Tokens would bypass the Authorization process, but the API Limits are reduced.
More information: Personal Access Tokens & Service Account Keys - Keap Developer Portal
As a bare minimum, you will find an example XML-RPC API code in the link below.
Although this is not REST, but you could apply the same principles in Delphi.
The Keap API Documentation will also help you if you get stuck.
Hope that helps.