If you select 'Custom' you need to edit your template file and add the following line just after the opening <body> tag: <?php if ( function_exists( 'gtm4wp_the_gtm_tag' ) ) { gtm4wp_the_gtm_tag(); } ?>
What you ask for, is files customization what in reference to Item Support Policy is not allowed. So if you want to modify files and don't know how, you should contact your web developer. Item Policy says: Item support does not include services to modify or extend the item beyond the original features, style, and functionality described on the item page. For customization services that will help you tailor the item to your specific requirements, we recommend contacting the author to see if they privately offer paid customisation services or checking out the great service providers on Envato Studio
Comments
What you ask for, is files customization what in reference to Item Support Policy is not allowed. So if you want to modify files and don't know how, you should contact your web developer. Item Policy says:
Item support does not include services to modify or extend the item beyond the original features, style, and functionality described on the item page. For customization services that will help you tailor the item to your specific requirements, we recommend contacting the author to see if they privately offer paid customisation services or checking out the great service providers on Envato Studio
thanks