It is important that autoLinkUser.IsApproved = true; is set to true, By design, the new user will be disabled by setting the autoLinkUser.IsApproved to true it will not be disabled.

AzureB2CBackofficeExternalLoginProviderOptions.cs

using Microsoft.Extensions.Options;
using Umbraco.Cms.Core;
using Umbraco.Cms.Web.BackOffice.Security;


namespace AzureAD
{
    public class AzureB2CBackofficeExternalLoginProviderOptions : IConfigureNamedOptions<BackOfficeExternalLoginProviderOptions>
    {
        public const string SchemeName = "OpenIdConnect";
        public void Configure(string name, BackOfficeExternalLoginProviderOptions options)
        {
            if (name != Constants.Security.BackOfficeExternalAuthenticationTypePrefix + SchemeName)
            {
                return;
            }

            Configure(options);
        }

        public void Configure(BackOfficeExternalLoginProviderOptions options)
        {
            options.ButtonStyle = "btn-danger";
            options.Icon = "fa fa-cloud";
            options.AutoLinkOptions = new ExternalSignInAutoLinkOptions(



                // must be true for auto-linking to be enabled
                autoLinkExternalAccount: true,

                // Optionally specify default user group, else
                // assign in the OnAutoLinking callback
                // (default is editor)
                defaultUserGroups: new[] { Constants.Security.AdminGroupAlias },

                // Optionally specify the default culture to create
                // the user as. If null it will use the default
                // culture defined in the web.config, or it can
                // be dynamically assigned in the OnAutoLinking
                // callback.

                defaultCulture: null,
                // Optionally you can disable the ability to link/unlink
                // manually from within the back office. Set this to false
                // if you don't want the user to unlink from this external
                // provider.
                allowManualLinking: false
            )
            {
                // Optional callback
                OnAutoLinking = (autoLinkUser, loginInfo) =>
                {
                    //This has to be set! Or else when the user is created it will be marked as disabled!

                    autoLinkUser.IsApproved = true;



                    // You can customize the user before it's linked.
                    // i.e. Modify the user's groups based on the Claims returned
                    // in the externalLogin info
                },
                OnExternalLogin = (user, loginInfo) =>
                {
                    // You can customize the user before it's saved whenever they have
                    // logged in with the external provider.
                    // i.e. Sync the user's name based on the Claims returned
                    // in the externalLogin info

                    return true; //returns a boolean indicating if sign in should continue or not.
                }
            };

            // Optionally you can disable the ability for users
            // to login with a username/password. If this is set
            // to true, it will disable username/password login
            // even if there are other external login providers installed.
            options.DenyLocalLogin = false;

            // Optionally choose to automatically redirect to the
            // external login provider so the user doesn't have
            // to click the login button. This is
            options.AutoRedirectLoginToExternalProvider = false;

        }
    }
}