There are three possibilities to get text into the left text field:
Using the keyboard as input instrument
Using the copy functions (3.3.2 - 3.3.6)
Using the command File -> Open... (3.2.2);
this will replace existing text.
If you want to keep existing text,
copy it with -> ( 4.3.1)
into the right text field (this will replace text there),
or copy it into the clipboard.
4.2 Right Text Field
There are three possibilities to get text into the left text field:
Using the keyboard as input instrument
Using the copy functions (3.3.2 - 3.3.6)
By clicking -> (4.3.1),
the left text field's content will be copied into the right text field
(and replace existing text).
To keep existing text, you can save the content of the right text field with
File -> Save (3.2.4).