So I'm building an application which exports contacts from various CRMs and unifies that data to run marketing reports on their contacts.

I was easily able to get HubSpot and InfusionSoft working with their Oauth2 apis, and I was hoping that salesforce would go just as easily.

My desired flow is to redirect the user from their private domain (e.g. user1.appDomain.com) to the salesforce site to log them in. Then I want to redirect them back to their private domain (user1.appDomain.com/oauthReturn) with their access token so I can start importing their data.

I registered an external application in my developer salesforce account, and have received my oauth tokens, but I need to know a few things (that googling hasn't solved yet).

So I need to know a few things first.

1. Will salesforce allow me to pass in the redirect url from the user's private domain?
2. Can I even do this with Oauth2 and php? The documentation is unnecessarily confusing, and most of the libraries are still using api tokens and all of the relevant documentation I can find is 2-3 years out of date.

If anyone knows where I should be looking to find these answers that would be extraordinarily helpful.

Thanks so much for your help!