We have an XML document released every Wednesday that contains all of the IP ranges for Azure Datacenters broken out by region. Please see https://www.microsoft.com/en-us/download/confirmation.aspx?id=41653:
This file contains the Compute IP address ranges (including SQL ranges) used by the Microsoft Azure Datacenters. A new xml file will be uploaded every Wednesday (Pacific Time) with the new planned IP address ranges. New IP address ranges will be effective on the following Monday (Pacific Time). Please download the new xml file and perform the necessary changes on your site before Monday.
The Hosted agent should be in the same region as your VSTS account, you need to whitelist the IP address ranges for your region which you can get from the link above. To verify your region in VSTS navigate to the Settings page at <account>.visualstudio.com/_admin/_home/settings, Under Account you will see a field for Region.