Recently, Microsoft announced an option in OWA to control the additional storage like Box addition in OWA, which allows users to add an attachment or Save a document in email to the third party storage options.
Microsoft Announcement on AdditionalStorageProvidersAvailable Parameter.
You can validate the change by running the below command.
Get-OwaMailboxPolicy “Policy Name” | fl additional*
If that parameter set as True, your users will have the below options to add third party storage accounts in OWA.
To restricted the third part storage accounts like Box or DropBox in OWA, disable that option by running the below command
Set-OWAMailboxPolicy “Policy Name” – AdditionalStorageProvidersAvailable:$false
Once that option disabled, user will not see the option to add third party storage option as shown below.
Make a note: OneDrive will not be considered as thirdparty storage option and earlier available command thirdpartyfileprovidersavailable will not work after 15th August 2019.