>The problem is: when I press a function key, the program executes the code I put behind it, but when my code has ended and focus returns to the calling textbox, then the sign 'Ÿ' is appended to the value that is originally in the textbox. Where does the 'Ÿ' comes from and how can I supress it ????

What function key you're refering to? Are you using ON KEY LABEL or KeyPress to launch your code on the function key pressing?