add additional activation to existing key

I have an existing key for Adobe Acrobat Pro DC 2017. The notes in our IT documentation says that this key is good for 4 activation's. I simply want to add another activation to this key. I want to be able to use this key for a 5th person. I am logged into the Adobe Licensing Website (LWS), I am looking at the information on this serial number but cannot see anywhere on the site or researching on the internet how to achieve this. I have always thought that Adobe licensing is confusing! Any help would be appreciated. Thank you,