How do I add a user attribute in a SAML SSO response - azure-ad-b2c

Question

I am working on using Azure AD B2C as the Identity Provider for a custom site that also provides SSO for a Blackboard Learn LMS site. Blackboard can do SSO through SAML, so I am using those capabilities within B2C. Unfortunately, SAML is not a protocol I am well-acquainted with, so my ability to troubleshoot is limited. I am working off of the tutorial at:
https://learn.microsoft.com/en-us/azure/active-directory-b2c/saml-service-provider?tabs=windows&pivots=b2c-custom-policy
Everything is going mostly pretty well, but I am trying to map user attributes that are shared with Blackboard so that when a new user is created, they have at least a basic profile. There are a couple attributes that are coming through naturally in this tutorial, but I am trying to specify the username for the profile (specs require that it is the email address), and I am not having any luck.
I added the OutputClaim named "userPrincipalName" in the SignUpOrSignIn policy (see code below), as it has the appropriate value coming from B2C, but according to a Blackboard test page, it is not among the values that are being sent in the SAML response (see screenshot further down).
...
<RelyingParty>
<DefaultUserJourney ReferenceId="SignUpOrSignIn" />
<TechnicalProfile Id="PolicyProfile">
<DisplayName>PolicyProfile</DisplayName>
<Protocol Name="SAML2"/>
<OutputClaims>
<OutputClaim ClaimTypeReferenceId="displayName" />
<OutputClaim ClaimTypeReferenceId="givenName" />
<OutputClaim ClaimTypeReferenceId="surname" />
<OutputClaim ClaimTypeReferenceId="userPrincipalName" />
<OutputClaim ClaimTypeReferenceId="email" DefaultValue="" />
<OutputClaim ClaimTypeReferenceId="identityProvider" DefaultValue="" />
<OutputClaim ClaimTypeReferenceId="objectId" PartnerClaimType="objectId"/>
</OutputClaims>
<SubjectNamingInfo ClaimType="objectId" ExcludeAsClaim="true"/>
</TechnicalProfile>
</RelyingParty>
...
Are there further steps that need to happen? I have tinkered with the TrustFrameworkExtensions (and Base) here an there, as well, but to no avail, and I can't find any described process for adding new attributes to the SAML response. Does anyone have an example/tutorial that could walk me through it?
(I looked at this question and an answer indicates that I might need to change the NameIdFormat. I can see that the format in my B2C app metadata is incorrect as it suggests, but I'm not sure how I would go about changing it)
Update
Based on #JasSuri-MSFT's suggestion, I set a (non-empty) default value for the UPN, and sure enough, that default value came through. So it would seem that the problem is that the UPN is null.
This is strange, as in the edit page for the user in B2C, there is clearly a UPN value. It also highlights another oddity that I had not mentioned. But there are other OutputClaims listed in that policy that are not being included either. One of them, email, I explicitly provided a value for, and it is apparently presenting as null as well.
Clearly there is another piece in the plumbing somewhere that I don't have established.

Answer 1

Add signInNames.emailAddress as an output claim in AAD-UserReadUsingObjectId and add signInNames.emailAddress as an output claim in the RelyingParty section. Use a partnerClaimType in the output claim in the RelyingParty to issue the claim with the name you like into the SAML Response.
<OutputClaim ClaimTypeReferenceId="signInNames.emailAddress" PartnerClaimType="email"/>
https://github.com/azure-ad-b2c/azureadb2ccommunity.io/wiki/LocalAccount-Sign-In-and-Sign-Up-policy
https://learn.microsoft.com/en-us/azure/active-directory-b2c/user-profile-attributes#identities-attribute
https://learn.microsoft.com/en-us/azure/active-directory-b2c/active-directory-technical-profile#outputclaims