The documentation clearly describes:
"confirmed": false, // Is set to true if this message was a confirmed message
However, I’m quite sure that in that case the network server (so: TTN) will send the acknowledgement for a confirmed uplink, without you having to do anything. If your application happens to be sending a downlink too (following the same documentation) then TTN will “piggy-back” on that downlink to send the ACK. But if there’s no such downlink scheduled, then TTN will create a downlink by itself.
That said: note that downlinks are bad for the network capacity and you can send at most 10 downlinks, including confirmed uplinks, per day. Sending some “true filed” for each uplink would be bad practice (even when assuming you’re not actually planning to use text for that).