There were some bugs with the 10.2.0 version of the JSON Feature parser. The known issues were fixed in version 10.2.1, and so it should be able to read an array of features like the sample below, or you can omit the square brackets and send an individual feature. I noticed that in your posted sample JSON, you were missing a curly bracket "}" near the end. Your screenshot shows a GeoEvent Definition that does not match the JSON data, though. It should have 2 fields, a Geometry and a String field. The geometry field can be named anything you like, but need to have the GEOMETRY tag on it. The string field needs to have the name "Alpha" in order to get the parser to populate the field correctly. Hope that helps.
[
{
"geometry":
{
"rings":
[
[10.114760188242185,49.44835148727626,0],
[10.114760188242385,49.44835148727626,0],
[10.114760188242085,49.44835148727326,0],
[10.114760188242485,49.44835148727426,0],
[10.114760188242185,49.44835148727626,0]
],
"spatialReference":
{
"wkid":4326
}
},
"attributes":
{
"Alpha":"alpha"
}
}
]