How to retrieve the userid from userprincipalname in azure active directory? - azure-active-directory

Question

My client has synchronization set up between an on-premises Active Directory (AD) and Azure Active Directory (AAD).
I am able to retrieve user information from AAD using Microsoft Graph without a problem but, I specifically need to get the AD UserID, ie ({domain}/{userid}).
I tried calling https://graph.microsoft.com/v1.0/users/firstname.lastname#domain.com/$select=userId but it did not work.
My questions are, is it possible? And in that case what is the actual attribute name? I have been looking around but haven´t been able to find a complete list of attributes.
EDIT:
After receiving one answer from Marilee I am including the C# code I have been using, ish. Both the calls do work for receiving user information from AAD, but not the AD UserID, ie ({domain}/{userid}) that I am looking for.
Attempt no 1
var requestUri = GraphBaseUri + $"/v1.0/users/{upn}?$select=userId";
var response = await _httpClient.GetAsync(new AuthenticationHeaderValue("Bearer", accessToken), requestUri).ConfigureAwait(false);
var content = await response.Content.ReadAsStringAsync().ConfigureAwait(false);
dynamic responseObj = JsonConvert.DeserializeObject(content) as JObject;
return responseObj.UserId; //NOT WORKING
Attempt no 2
var graphClient = new GraphServiceClient(new DelegateAuthenticationProvider((requestMessage) => {
requestMessage
.Headers
.Authorization = new AuthenticationHeaderValue("Bearer", accessToken);
return Task.FromResult(0);
}));
// Retrieve a user by userPrincipalName
var user = await graphClient
.Users[upn]
.Request()
.GetAsync();
return user.ObjectId; //NOT WORKING

Answer 1

The attribute you're referring to is the objectID. From Graph API you can use UPN like you said:
GET /users/{id | userPrincipalName}
You can look up the user in a few different ways. From the /users endpoint you can either use their id (the GUID assigned to each account) or their userPrincipalName (their email alias for the default domain):
// Retrieve a user by id
var user = await graphClient
.Users["00000000-0000-0000-0000-000000000000"]
.Request()
.GetAsync();
// Retrieve a user by userPrincipalName
var user = await graphClient
.Users["user#tenant.onmicrosoft.com"]
.Request()
.GetAsync();
If you're using either the Authorization Code or Implicit OAuth grants, you can also look up the user who authenticated via the /me endpoint:
var user = await graphClient
.Me
.Request()
.GetAsync();
From Powershell you can query Object IDs:
$(Get-AzureADUser -Filter "UserPrincipalName eq 'myuser#consoso.com'").ObjectId

Answer 2

The way to do this is;
var requestUri = GraphBaseUri + $"/v1.0/users/{upn}?$select=onPremisesSamAccountName";
var response = await _httpClient.GetAsync(new AuthenticationHeaderValue("Bearer", accessToken), requestUri).ConfigureAwait(false);
var content = await response.Content.ReadAsStringAsync().ConfigureAwait(false);
var responseObj = JsonConvert.DeserializeObject<ResponseObj>(content);
return responseObj.OnPremisesSamAccountName;
Notice the select where we collect the onPremisesSamAccountName. Though, I haven´t found a comprehensive list of all attributes possible to retrieve which would have been nice.