how to publish my intranet as a sub page without affecting the ssl

hi everybody.
I have a question on publishing my subdomain to my domain with SSL. Right now i have one domain name. for example www.mydomain.com, it is under mySecureSign Inc. Secure Site.I’d like to publish my intranet http://global/webintra outside as sub page of mydomain.com for example: www.mydomain.com/webintra
Please help me on how to configure it securely or maybe when www.mydomain.com/webintra access over the web that will require a windows authentication from my Active directory.

It’s my first time to do this. Please help me guys. Thanks.