best is check the API documentation :
https://developer.cisco.com/docs/ftd-api-reference/latest/introduction-to-firepower-threat-defense-rest-api/
have you tried explorer :
https://<FDM_IP_or_name>:<https_port>/#/api/api-explorer/
what are you looking using SPI to make S2S VPN configuration ?
as per i know FMC have that option, not sure Latest FDM may have if you running 7.2 or above code.