Get Customer Resource Id with Multi Tenant OAuth on Microsoft Dynamics - oauth-2.0

Question

I want our my users who are using online Microsoft dynamics to give us access to their account through OAuth2. To be clear, my users live in their own tenants and NOT part of my tenant.
So I registered an Azure AD application and made it multi-tenant but the authorize URL required the resource id which is the exact customer URL on MS Dynamics. like
https://{orgid}.crm.dynamics.com
but I do not want the user to enter their URL manually. I want to automatically figure out their resource id during the OAuth process and complete the process.
how can I do so?
Btw, I am not using C# and I would appreciate it if the HTTP calls could be provided.

Answer 1

Recently I have been working/wrestling with some multi-tenant web apps that need to access D365 via OAuth2. While I do not claim to have mastered it, I have gotten a few such apps working.
Here are some thoughts for you:
I have not attempted to put the tenantId into the CRM url. To construct the CRM url I use the org "url name", which you can get using the below code.
This code assumes that you've put the base URL into the appSettings node of the web.config:
<add key="ida:OrganizationHostName" value="https://{0}.crm.dynamics.com" />
Then this code might should help you along:
private string getOrgName(string user)
{
var a = user.IndexOf('#');
var b = user.IndexOf('.');
return user.Substring(a + 1, b - (a + 1));
}
var signedInUserID = ClaimsPrincipal.Current.FindFirst(System.Security.Claims.ClaimTypes.NameIdentifier).Value;
var tenantID = ClaimsPrincipal.Current.FindFirst("http://schemas.microsoft.com/identity/claims/tenantid").Value;
var userObjectID = ClaimsPrincipal.Current.FindFirst("http://schemas.microsoft.com/identity/claims/objectidentifier").Value;
var OrganizationHostName = ConfigurationManager.AppSettings["ida:OrganizationHostName"];
var organizationName = getOrgName(User.Identity.Name);
var resource = string.Format(OrganizationHostName, organizationName);
Where I have seen the tenant Id used is when establishing the auth context
var authContext = new AuthenticationContext($"https://login.windows.net/{tenantID}");
As shown above you can get it from the ClaimsPrincipal, but I think my apps are all now using https://login.windows.net/common rather than appending the tenantId.
If you want to get fancy you can query the Discovery service to see what orgs a user has access to. For me, Colin Vermander's article was the key to getting that working.