DevOps Build. How to build a multi-project solution (Windows Service) that has a windows forms test harness

We have an old Windows service I am trying to move to GIT and DevOps (on prem). The service project builds off of 3 other projects in the solution however there is a 5th project which is a windows forms app that we use to develop and debug. I would like to set up a build where the service is built using the latest copies of the 3 source projects and keep the windows form project in the solution but not affecting the build. To me this is probably similar to a MS Test or NUnit project on a web API through DevOps but we have been using Postman and keeping them apart. Any help on how to solve would be great

Full Solution

Windows Service Project has a project dependency on Windows DLL 1 which is built using a dependency on -- Windows DLL 2 and a third project that is essentially a collection of XSL templates.

This is what I want to build and deploy. Additionally there is

Windows form Test Project (Current default start up project) that we use to develop and debug locally

I could I suppose have 2 solutions in GIT and DEVOps but if I can do this simply in one that would be very helpful. I think anyone with test projects in the DevOps solution can provide the answer

Thank you.