How to add .txt name to save in SD

i have Arduino and an SD card which is working perfectly.

how can i create my own file ?

what i mean is, if i send "Test" through serial monitor

how can i convert "test" into test.txt and save it to SD??

any idea?

void receiveEvent(int howMany)
{
   int i = 0;
  while(Wire.available() > 0)
  {
    message[i] = Wire.read();
    i++;
  }
  lcd.print(message);
  myFile = SD.open(message, FILE_WRITE); //How can i convert the content of message to .txt to save in SD? please help.
  myFile.close();

}

strcat()?

http://www.cplusplus.com/reference/cstring/strcat/