C# best practice for securing API keys in program using the Generated API

The Generated API for C# targets net45 and netcore2.0
As such I am unable to use the current best practice for storing user secrets in my API program, as the targeting of net45 seemingly disallows Microsoft.Extensions.Configuration.UserSecrets
What do you recommend as best practice for storing the API keys in a console application in a Development environment?
app.config? That does not seem a very good practice