Hello,
By default, custom email only works with UMP forms. However, please add following code to your functions.php to hook custom email with user_register action.

add_action( 'user_register', 'umpRegistrationEmail' );
function umpRegistrationEmail( $user_id ) {
    global $userMeta; 

    $user = new WP_User( $user_id );
    $key  = 'registration';
           
    $data = $userMeta->getEmailsData( $key );        
    $role = $userMeta->getUserRole( $user->ID );

    if ( empty( $data['admin_email']['um_disable'] ) ) {
        $mailData = @$data['admin_email'][ $role ];
        $mailData['email']        = !empty( $data['admin_email']['um_all_admin'] ) ? $userMeta->getAllAdminEmail() : get_bloginfo( 'admin_email' );
        $mailData['email_type']   = $key;
        $mailData['receipt_type'] = 'admin';
        
        $mailData = apply_filters( 'user_meta_raw_email', $mailData );
        $mailData['subject']  = $userMeta->convertUserContent( $user, @$mailData['subject'] );
        $mailData['body']     = $userMeta->convertUserContent( $user, @$mailData['body'] );        
        $userMeta->sendEmail( $mailData, $user ); 
    }

    if ( empty( $data[ 'user_email' ][ 'um_disable' ] ) ) {
        $mailData = @$data['user_email'][ $role ];
        $mailData['email']        = $user->user_email;
        $mailData['email_type']   = $key;
        $mailData['receipt_type'] = 'user';
        
        $mailData = apply_filters( 'user_meta_raw_email', $mailData );
        $mailData['subject']  = $userMeta->convertUserContent( $user, @$mailData['subject'] );
        $mailData['body']     = $userMeta->convertUserContent( $user, @$mailData['body'] );        
        $userMeta->sendEmail( $mailData, $user ); 
    }
}