how to insert a extra line in a text file

how to insert a extra line in a text file using a sh command

iam trying to think of a way to add a extra line but without deleting the whole text

do anyone have any ideas

echo "" >> mytextfile

one way to add a line at the end.

iam trying to let the user enter a line of text and the entered line if text will be inserted at the bottom of the text file