Menu with target option (to link an iframe)

Just a guess, but I think what you want is an ‘anchor link’. Here is an article that may help: https://help.typepad.com/anchor-tags.html

No, I don’t want to do an anchor link (that is easy), I want to link to an iframe.

I think you can use a Walker to do what you want. You would code the URL like this:

https://yourdomain.com/optionalpage/target/iframename

Add the walker to wp_nav_menu() similar to this:

wp_nav_menu( array( 'container_class' => 'menu-header', 'theme_location' => 'primary', 'walker' => new iframe_walker() ) );

And define the walker in functions.php:

/* Convert "https://mydomain.com/pagename/target/iframename" to
 * href="https://mydomain.com/pagename/" target="iframename"
 *
 * The walker must be added to wp_nav_menu() in header.php
 * */
class iframe_walker extends Walker_Nav_Menu
{
      function start_el(&$output, $item, $depth, $args)
      {
            global $wp_query;
            $indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

            $class_names = $value = '';

            $classes = empty( $item->classes ) ? array() : (array) $item->classes;

            $class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item ) );
            $class_names = ' class="'. esc_attr( $class_names ) . '"';

            $output .= $indent . '<li id="menu-item-'. $item->ID . '"' . $value . $class_names .'>';

            if ( preg_match( '/^(.+?)\/target\/(.+)$/', $item->url, $matches ) ) {
               $item->url = $matches[1] . '/';
               $item->target = $matches[2];
            }
            $attributes  = ! empty( $item->attr_title ) ? ' title="'  . esc_attr( $item->attr_title ) .'"' : '';
            $attributes .= ! empty( $item->target )     ? ' target="' . esc_attr( $item->target     ) .'"' : '';
            $attributes .= ! empty( $item->xfn )        ? ' rel="'    . esc_attr( $item->xfn        ) .'"' : '';
            $attributes .= ! empty( $item->url )        ? ' href="'   . esc_attr( $item->url        ) .'"' : '';

            $item_output = $args->before;
            $item_output .= '<a'. $attributes .'>';
            $item_output .= $args->link_before .$prepend.apply_filters( 'the_title', $item->title, $item->ID ).$append;
            //$item_output .= $description.$args->link_after;
            $item_output .= '</a>';
            $item_output .= $args->after;

            $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
            }
}

Thank you!
It almost works! but I have a problem: My link is this way> https://mydomain.com/pagename.html, not this way: https://mydomain.com/pagename/
Can I change something to make it works?

Try changing this line:

$item->url = $matches[1] . '/';

to this:

$item->url = $matches[1];

Yeah!!!!!!!!! ??
Now it works perfect!
Thank you very much for your help!