In Visual Studio 2017 Enterprise, assigning RunSettings files can typically be done here, under "Test" > "Test Settings":
However, I don't see anything relevant when going to "Test" on Visual Studio 2019 Enterprise (Preview 4.0):
Where can I assign the RunSettings file to run with in the Visual Studio 2019 Enterprise IDE?
The setting was changed in VS 2019 - you can find it in Test Explorer tool Window
See the docs for the exact instructions.