Text to be placed on the page is entered in the edit-box by simply typing-in the string. There are also some features for making repetitive tasks quicker:

If other fields on the page have been configured to use addtext already, the string last entered is given in the edit box automatically.
Tick the Single line check-box when filling in a series of one-line fields (such as a post code or account number). A single press of the Enter key then acts as if it were a click on the OK button (rather than starting a second line of text) and is quicker than clicking 'OK'.
It is set by default for any field whose height is insufficient for two lines in the selected font size.