If you’re using the Back To Parent broadcast plugin and want the checkbox always selected, you’ll have to use some javascript.

Below is a whole plugin that you can put in your plugins directory. Activate it and the checkbox will be automatically selected.

<?php
/*
Author:            edward_plainview
Author Email:    edward@plainviewplugins.com
Author URI:        https://broadcast.plainviewplugins.com
Description:    Selects the Back To Parent checkbox
Plugin Name:    B2P Select
Plugin URI:        https://broadcast.plainviewplugins.com/threewp-broadcast/
Version:        1
*/

add_action( 'admin_footer', 'b2p_checkbox' );

function b2p_checkbox()
{
    echo '<script type="text/javascript">
        jQuery( document ).ready( function($ )
        {
            $( "#plainview_sdk_broadcast_form2_inputs_checkbox_back_to_parent" ).attr( "checked", "checked" );
        } );
    </script>';
}