The first thing you need to decide before starting this process is what you would like your partner directory to be called. A subdomain will look like this:
partners.yourdomain.com OR
directory.yourdomain.com OR
agencies.yourdomain.com
The naming convention is completely up to you. The portion before the yourdomain.com can be customized and changed at any time.
Setting up your subdomain:
Create a CNAME record in your DNS and call it partners (or whatever you would like to call your directory)
Point that record to your PartnerPage URL such has yourdomain.partnerpage.io
Once this has been done, let the folks at PartnerPage know, and the team will add an SSL and complete a couple other items.
You can always email support@partnerpage.io with any questions.