add_action( 'mainwp_manage_sites_edit', 'jkws_mainwp_manage_sites_edit', 10, 1 );
function jkws_mainwp_manage_sites_edit( $website ) {
    $webhost  = apply_filters( 'mainwp_getwebsiteoptions', false, $website, 'webhost' );
    ?>
    <h3 class="ui dividing header">Additional information (Optional)</h3>
    <div class="ui grid field mainwp_addition_fields_addsite">
        <label class="six wide column middle aligned"><?php esc_html_e( 'Webhost', 'mainwp' ); ?></label>
        <div class="ui six wide column" data-tooltip="<?php esc_attr_e( 'Webhost', 'mainwp' ); ?>" data-inverted="" data-position="top left">
            <div class="ui left labeled input">
                <input type="text" id="mainwp_managesites_edit_webhost" name="mainwp_managesites_edit_webhost" value="<?php echo esc_html($webhost); ?>"/>
            </div>
        </div>
    </div>
    <?php
}

//  Update the Webhost field in the database of the edited site
add_action( 'mainwp_site_added', 'jkws_mainwp_site_added', 10, 2 );
function jkws_mainwp_site_added( $website, $information ) {
// do addition process with custom $_POST fields, for example: mainwp_managesites_edit_webhost
}