After testing a little bit, it looks like that specific website is using Akamai Ghost and has been configured to block the default go http package user agent.
The default user agent appears to be Go-http-client/1.1
If you change your user agent
req.Header.Set("User-Agent", "my-client-app")
The request will work. However, the website in question appears to not want to be programatically crawled. Perhaps you should respect their wishes.
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solved Sending Requests to Https site