The URL address is automatically created for you, it is created based off the title of the add-on, category, and the other elements you choose. You can modify the URL address easily by clicking on the lock symbol next to the link.
The URL is not the most important part of SEO but you should be paying close attention. If your URL is unique and formed by keywords it is better. All spaces are automatically filled with a hyphen. As your links will be shared, it is better that they are short and clear.