selecting the selected

[Richmond]

Umm . ..

so I have a field [called fld "TEKST"]full of text and I select some of 
it with my mouse

I then click on a button that contains the following script:

on mouseUp
     copy the selected of fld "TEKST"
end mouseUp

nix!

now, I've tried "the selectedText" and "the selectedChunk" to no avail . . .

Please advise