Problem reading a char or string

Ty for your replies

My goal is to enter my reference and when I reach 11 caracters ( xxx-xxx-xxx ), it saves my reference in referencePDU parameters so that I can use this parameter later

Using :

Serial.print(customKey);
referencePDU[n] = customKey;
n += 1;

I have :

Serial number of PDU:
192-538-765-
192538765

Getting closer ^^ But how could I have the - between each group of 3 numbers ?

Thanks

Thomas