You must separate YOUR App login from anything to do with Shopify. No one cares how you log people in to your App, but there should be zero concern for that coming from the oAuth pattern. You use oAuth to authenticate with Shopify, and that has zero to do with accounts logged into your App. So create an URL in your partner dashboard App setup that works for oAuth with Shopify. If you do anything to interfere with basic oAuth, you will fail, and that is not a good start!