XIAO SAMD21 - SoftwareSerial as send only

I have SoftwareSerial running.

The normal start is;

SoftwareSerial SS(RXpin, TXpin);

I only need the SoftwareSerial as transmit, so that only a single physical pin is used.

I tested it with TXpin set to A6 and RX pin set to -1, it seems to work, up to 19200 baud.

Is -1 for RX OK to do, or is there internally another un-used pin on the XIAO that you can use ?