I couldn’t find any documentation either but it can be accomplished with a quick and easy bash script:
token=curl -k "https://192.168.2.1/api/login?username=admin&password=admin" |grep token|cut -f4 -d\"`
curl -k -X POST -H "Content-Type: application/json" -d '{
"recipients":["15178765309"],
"message":"test messages are the best!!!!"
}' "https://192.168.2.1/api/sms/outbox/?token=$token"