Okay - Garmin hid this pretty well - but I managed to find the following example:
{
"Messages":[{
"Message":"String content",
"Recipients":[9223372036854775807],
"ReferencePoint":{
"Altitude":9223372036854775807,
"Coordinate":{
"Latitude":1.26743233E+15,
"Longitude":1.26743233E+15
},
"Course":9223372036854775807,
"Label":"String content",
"LocationType":0,
"Speed":9223372036854775807
},
"Sender":"String content",
"Timestamp":"\/Date(928149600000+0000)\/"
}]
}
I needed to pass the recipient IMEI in an array, and the timestamp as above - at last! Success! It worked
Thanks @djangelic and @MutedJam for the support!!