Use uart.send() to send data in the started UART.
// Javascript Example
var uart = obniz.getFreeUart();
uart.start({tx: 0, rx: 1, baud:9600 });
uart.send("Hello");
uart.send([0x00, 0x01, 0x02]);
uart.send() does not wait for sending to be finished. UART also does not check if the other party has received the signal, so it will not result in error even if it is not connected with the other party.