Hi,
Thanks for taking the time to provide your feedback. Remote download functionality is intentionally blocked in v6.0 and later for security reasons:

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CORE-8544 - Discontinue support for remotely hosted downloads in download files
http://docs.whmcs.com/Changelog:WHMCS_V6.0

It would be necessary to upload the file to your /downloads directory and enter the filename in the Filename field.
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