In my [Plesk web admin edition](https://support.plesk.com/hc/en-us/articles/115002234393-How-to-enable-HTTP-Strict-Transport-Security-HSTS-for-a-domain-on-the-Plesk-server-) I just activated HSTS on my main domain `www.domain.tld` with

```
add_header Strict-Transport-Security "max-age=31536000; includeSubDomains" always;

```

The test on [ssllabs.com](https://www.ssllabs.com/ssltest/) says that everything works fine. The problem is my subdomain (`subdomain.domain.tld`). If I test my subdomain on ssllabs it says that there is no HSTS activated.

Should I include the header

```
add_header Strict-Transport-Security "max-age=31536000; includeSubDomains" always;

```

on my subdomain too or is an implementation on my main domain sufficient?

I thought by adding `includeSubDomains` there was no need for adding it explicitly on subdomains.