Spanning Backup for Office 365 offers a public API, in which you can automate license assignment. If you have a large number of Spanning licenses to assign at once, using PowerShell to upload a CSV to mass-assign all of those licenses will be more efficient.
Spanning has an un-official example PowerShell module that you can use to complete this process, which can be found here. Additionally, please watch this instructional video below, which details how to use this example PowerShell module:
If you have any additional questions, please contact Spanning Support - we're always happy to help!