You do not need to manually upload the trial license. It is built-in. The trial period begins the first time you start FortiWeb-VM.
Once the trial expires, most functionality is disabled. You need to purchase a license to continue using FortiWeb-VM.
AWS BYOL FortiWeb-VM deployments do not include the free trial license. Instead, you can evaulate FortiWeb using the on-demand/hourly version from AWS.