Final Solution to enable in-frame Monit GUI

Assuming that ISPConfig v3.2.5 is installed on ispconfig.domain.tld, add these additional lines in file
/etc/apache2/sites-available/ispconfig.vhost

Code:

<IfModule mod_headers.c>
# Enable Monit GUI
Header always append X-Frame-Options SAMEORIGIN
Header set Content-Security-Policy "frame-ancestors 'self' domain.tld"
# Enable Monit GUI
</IfModule>

systemctl reload apache2.service. Done.

Now, even newbie can enjoy in-frame Monit GUI.