If you are using FortiADC for http to https ssl offloading, adding x-forward-proto http header is required for some applications
You can add x-forward-proto:https header via this way;
- Go to Server Load Balance -> Virtual Server -> Content Rewriting menu and click Create New

- Fill the sections as; Name: x-forward-proto, Action: Add HTTP Header, Header Name: X-Forwarded-Proto, Header Value: https

- Go the Server Load Balancing -> Virtual Server -> Virtual Server menu and select your Virtual server that x-forward-proto will ve enabled;
- At the Basic section enable Content Rewriting than select x-forward-proto from Available Items and put this item to the Selected Items area. Then Save this Virtual Server settings and exit.

After this configuration the x-fortward-proto will be added the http header by NLB.