https://tools.ietf.org/html/rfc5883#ref-BFD-1HOP
Under a static route:
set routing-options static route 192.168.47.0/24 next-hop 172.16.1.2
set routing-options static route 192.168.47.0/24 bfd-liveness-detection minimum-interval 1000
set routing-options static route 192.168.47.0/24 bfd-liveness-detection description Site-xxx
set protocols bfd traceoptions file bfd-trace
set protocols bfd traceoptions flag all
Under an protocol: