Performance Testing

I am using javascript, but the logic is the same - you could try to get the length of the string, then obtain a random starting point between 1 and the length of your string (*minus 6, so it won't grab less than 6 characters) and use it in the aqString.SubString method [referenced here]:

function getRandomString() {
  var string = "ABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890abcdefghijklmnopqrstuvwxyz";
  var strCount = aqString.GetLength(string); // length of string
  var strMax = strCount - 6; // strCount - 6 >> so it doesn't grab less than 6 characters
  var randoStart = Math.round(Math.random()*(strMax - 1) + 1); // random # between 1 and strMax
  var randoStr = aqString.SubString(string, randoStart, 6)
  Log.Message(randoStr);
  return randoStr;
}

function test() {
  getRandomString(); // 1 
  getRandomString(); // 2
  getRandomString(); // 3
}

 Output:

You can use the TestComplete string generator

try one of these

https://support.smartbear.com/testcomplete/docs/testing-with/data-driven/generators/types/string.html

https://support.smartbear.com/testcomplete/docs/testing-with/data-driven/generators/types/custom-string/about.html