If you know of a license for this version, you can add it a number of ways.
If you have several licenses to add, you can edit the package using the "Edit Package" button (in the upper right hand corner of the screenshot above) which will bring up the dependency editor.
If you only have a single license to add you can also click the "Add License" button lower on the dependency page and skip the dependency editor.
From here you can select the Add a license group which will allow you to add a license group:
you select the license here and click confirm and you will be taken to the screen to document the license location. You can insert the URL to the license in the URL field:
Here you can add the URL to the license and make any other changes to license text or title and click confirm.
The license will be part of the package now but you will need to rescan any affected projects for the license to be updated.