This is not so trivial if the size is big. (Don't operate with your 7, you can make the interface accepting any append). The common mistake is working with whole text which can be too slow.
You can find correct code compared with incorrect code and explanation in my Solution to this Question:
copy + paste into/ within a rich textbox?[
^].
(The only difference with your case: instead of Clipboard text, use you input parameter, the text to be appended.)
—SA