Hello @edu_horse,

To insert the invisible tag you have to create and activate the child theme.

Child theme references:

https://codex.wordpress.org/Child_Themes

You can also create the child theme using plugin given below:
https://wordpress.org/plugins/wp-child-theme-generator/

After creating and activating the child theme , in your child themes functions.php paste below given code.

https://gist.github.com/anonymous/00affa905bba07f543f562d0a92cce0d

Hope this helps.

Best regards!!