I researched the best way to add users GitHub OAuth2 Apps programmatically, but you can only do that manually and each user needs their own OAuth2 App. A way to do it is to build your own OpenID Connect Provider with for example Ory's Hydra.
Another way which for me is simpler and that we need to build anyway is to build a web app (SPA) where users log in and click a button which takes them thru a workflow that saves the type of service selected (CPU, RAM, etc) and users info such as GitHub credentials in a secure way (which they enter after they created the GitHub OAuth2 App) on a managed RDB such as DynamoDB or Firestore. This way there is no need for extra servers and databases and to learn Hydra. You only have to use your tremendous React and Node.js skills.