Update entra-id-saml.mdx

Author: clairekinde11

src/content/docs/authenticate/enterprise-connections/entra-id-saml.mdx

@@ -58,7 +58,7 @@ You can make a connection available only to a specific organization, or you can
 
    </Aside>
 
-2. For the **Entity ID**, enter a random string like `hEb876ZZlkg99Dwat64Mnbvyh129`. Make a copy of the string as you will add this to your SAML application later.
+2. For the **Entity ID**, enter a random string like `hEb876ZZlkg99Dwat64Mnbvyh129`. Make a copy of the string as you will add this to your SAML application later. Note that some older Entra ID tenants require the Entity ID to have a prefix of `spn:` If your connection fails, this could be why.
 3. Scroll past the IdP metadata URL and other key attribute fields. We will add this information later. 
 4. Enter **Home realm domains**. This speeds up the sign in process for users of those domains.
    Note that all home realm domains must be unique across all connections in an environment. For more information about how, see [Home realm domains or IdP discovery](/authenticate/enterprise-connections/home-realm-discovery/).