How to Connect to SharePoint Site via REST interface.

Hi,

I'm writing a C# application that connects to an on-premise SharePoint implementation.  I am looking to use the REST interface and follow the guidance listed below.

https://docs.microsoft.com/en-us/sharepoint/dev/sp-add-ins/complete-basic-operations-using-sharepoint-rest-endpoints

https://docs.microsoft.com/en-us/sharepoint/dev/sp-add-ins/get-to-know-the-sharepoint-rest-service?tabs=csom

However, I am stuck on establishing the initial connection to get the "accesstoken".  

Does anyone know how you do this with REST and not with ClientContext or any SP libraries?

Is this possible?

Thanks - Greg.