ty danielmitterdorfer 
but can you give me the syntax ?
I have to declare in the mapping a new field (that doesn't exist in my JSON message 'incoming'), like this ?
"properties": {
"my field x": {...},
"my field y" :{...},
...
"indexing_ts": { "type": "date", "format": "strict_date_optional_time||epoch_millis"}
}
??
Is it enough ? How can it get the current timestamp (now) ?