Add this function in your theme’s functions.php
function my_enqueue($hook) {
if ('post.php' != $hook ) {
return;
}
wp_enqueue_script('my_custom_script', get_template_directory_uri() . '/js/myscript.js');
}
add_action('admin_enqueue_scripts', 'my_enqueue');`
In myscript.js place this code
jQuery(document).ready(function(){
jQuery('#wp-link-target').prop("checked", true);
})
It worked for me.