The custom vault type feature that is currently being developed will likely provide the functionality that you need.
In the meantime, you can use the work-around suggested by @JeremyCouch (storing the URL in a custom text field), or you can consider storing your secure notes in the “Notes” section of a login item (with a blank username and password) instead of using a Secure Notes item. The advantage of using a Login item to store the URL is that you will be able to use the “Launch” button to open the URL from the Bitwarden app.