In a post a while back I detailed how to create a new Office 365 tenant using an Open License key. You’ll find that post here:
So what happens when you want to add an Open License key to an existing Office 365 tenant? basically the same process, you just click a another link.
First step after you have your key is to visit is to visit:
Here you will see a screen like:
If you are going to create a new tenant using the key you have obtained you select the option on the left as detailed in the previous post. However, if you already have an existing tenant you wish to apply the key to you select the link on the right.
You select the Sign in link and then login as an administrator to your existing Office 365 tenant.
You then enter product key, where it will be verified.
Once you accept this you’ll be done and can access the features the new product key enables.