Cant append new line to an existing .txt file, instead it creates a completely new .txt file

Hello, so i am trying to create a character list inside a text file that has been created before when making an account, the problem that i am having is with my script, because it is creating a completely new text file with the character names, but i want them to be inside the account text file

here is the code:
if (Charactername != “”) {
CN = true;

				if (CN == true) {
					
					
					charactername.GetComponent<InputField> ().text = "";
					string createText = Charactername + Environment.NewLine;
					//string [] charlist = {Charactername};
					System.IO.File.WriteAllText(@"C:/UnityTestFolder/" + Username + ".txt", createText);
					
					print ("Creation completed.");
				} 

`

From the documentation for WriteAllText

“Creates a new file, write the
contents to the file, and then closes
the file. If the target file already
exists, it is overwritten.”

You probably want to use AppendAllText instead.