Upgrade Insecure Requests via .htaccess or meta tag to prevent mixed content

.htaccess

<ifModule mod_headers.c>
Header always set Content-Security-Policy "upgrade-insecure-requests;"
</IfModule>

meta.html

<meta http-equiv="Content-Security-Policy" content="upgrade-insecure-requests">

plesk.panel

add_header Content-Security-Policy upgrade-insecure-requests;