Hi,
You can use FMC virtual in evaluation mode but to manage FTD we need to switch the FMC to smart licensing as Smart account is mandatory for FTD. Once you switch to smart licensing then the license need to be added under the account.
If you are a Cisco partner, then you can request trial/evaluation license for FTD and FMC both by opening the case through Cisco GVE Team. If not a partner, then send email to licensing@cisco.com to request for trial license or contact your preffered partner/Cisco SE/Cisco AM.
I would suggest you to create a smart account if you are not having. Register your FMCv with it and add your FTD's to FMCv. You will notice license compliance warnings and alerts in your smart account which can be fixed by trial license for 60 days and can be renewed again if you request them.