Can you please clarify your version of FortiGate firmware? As far as I am aware, no such functionality is available even for SSL-VPN, so this claim is surprising. There is always a chance that I missed something, so if you have any documentation mentioning this, I am eager to be enlightened. :)
I am also curious about what is your goal with this. It certainly won't let you dodge the issue of your chosen certificate not being issued for the IP address being initially requested and triggering a certificate error, as the full TLS handshake must finish before the HTTP-based redirect can happen.
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