You can enter the release key using the API command below through either the web interface under Configuration > API or using SSH. However, after TC6.1, release keys aren't needed anymore for software upgrades, which is why they removed the option to input release keys directly via the web interface. So starting with TC7, if for some reason a release key is needed to be added, it must be done through the API.
xCommand SystemUnit ReleaseKey Add Key : <release key>
If you searched the forums, you would have found it's been asked a few times already.
how-add-release-key-sx20-tc710
sx20-release-key
