Thanks for writing in! If you're having an issue getting the purchase code to be accepted in the backend on your website, it could be something environment related keeping that API from communicating with our systems. However, you should still be able to manage your validation from the licenses page you previously mentioned.
To do so, go to your licenses page, and find the licenses that you're wanting to assign to a URL. Next, under the Sites column in that row, find the Production input and click the pencil icon to the right of that input (when you hover over it, an Edit tooltip should appear above). Once you've clicked that pencil icon, the input itself will become active and focused, and you will be able to input your website's URL for product validation. The URL you enter here should match your website URL entered into the settings of your WordPress installation (you can find this under Settings → General → Site Address (URL) in your WordPress admin). Once you are done entering in your URL into the input on your licenses page, click the check mark to save your changes.
After all of that is complete, your website should be validated and you'll be good to go! Hopefully that helps to provide a little more context and get you moving in the right direction. Cheers!