NeilSM

Documentation for InsertLine says:

InsertLine
Opens the specified file and inserts the contents at the specified line number.
This operation will not overwrite any existing content at the specifid (sic) line.

Should say:
Opens the specified file and inserts the contents before the specified line number.
This operation will not overwrite any existing content at the specified line.
If the selected line number is greater than the number of lines in the file the line will be appended.
If the selected line number is 0 the line will be ignored - no alteration.

Example:
my_return = File.InsertLine("C:\my_file.txt",1,"This is a test " + 1)
TextWindow.WriteLine(my_return)
my_line = File.ReadLine("C:\my_file.txt",1)
TextWindow.WriteLine(my_line)

Just type the first one or two letters of the word, scroll up or down to the desired (key)word and press TAB.