Fortinet removed the feature to inject credentials in FortiClient 5.4 and above. However, they left the feature with FortiSSL.
Company terminology:
Fortinet: Name of the company
Fortigate: Fortinet router
Forticlient and FortiSSL: Fortinet VPNs
The CLI for FortiSSL is not included anymore with the download of FortiClient. You need to download the tools related to the current version of FortiClient.
The FortiSSLVPNClient.exe file needs to be saved in the installation folder of FortiClient.
You need to have a support plan with Fortinet to download the tools. If you do not have a support plan, try to contact Fortinet directly and ask them to send you the file.