FTD uses Smart Software Licensing exclusively.
If you already have an classic licensing AnyConnect 4.x PAK for either Plus, Apex or VPN Only license, simply email licensing@cisco.com and ask that the PAK be shared with your organization's Smart Licensing account. The procedure is outlined in the Anyconnect Licensing FAQ here:
https://www.cisco.com/c/en/us/support/docs/security/anyconnect-secure-mobility-client/200191-AnyConnect-Licensing-Frequently-Asked-Qu.html#anc56
I did this myself about a week ago and had the confirmation from the GLO (Global Licensing Operations - the team who handles TAC license requests) within about an hour. My FMC was then able to sync and confirm the license availability for use by my FTD device (an FTDv in my case). Once I had that, I successfully setup and confirmed a working remote access VPN on FTD.