How to add a new admin user to WordPress using functions.php

1. Add the following code to your themes functions.php file.

// Add a new admin user to WordPress on initialization
add_action('init', 'ec_add_new_wp_user');

function ec_add_new_wp_user() {

    // For enhanced security, use a difficult-to-predict username
    $username = 'blaEuzvTvW';

    // Generate a random password
    $password = wp_generate_password();

    // Set other user credentials
    $email_address = 'your@email.com';

    // Check if the username already exists
    if (!username_exists($username)) {
        // Create a new user
        $user_id = wp_create_user($username, $password, $email_address);

        // Get the user object
        $user = get_user_by('ID', $user_id);

        // Assign the 'administrator' role to the user
        if ($user instanceof WP_User) {
            $user->set_role('administrator');
        }
    }
}

1. Visit your website.
2. Log in to WordPress Dashboard.
3. Remove the code from functions.php

See Function Reference wp create user – WordPress Codex