You can actually run all the Openstack CLI's in debug mode to see the cURL commands it's executing.

for example you could run nova --debug volume-attach $UUIDofServer $UUIDofVolume auto and see what API calls it's making then just translate the HTTP Verbs, Body and syntax to your REST API client.

[root@node-3 ~]# nova help volume-attach
usage: nova volume-attach <server> <volume> [<device>]

Attach a volume to a server.

Positional arguments:
  <server>  Name or ID of server.
  <volume>  ID of the volume to attach.
  <device>  Name of the device e.g. /dev/vdb. Use "auto" for autoassign (if
        supported)

Here what you are searching for...