hi there, for everyone interested about it, i used the following solution:
as fox said, in fieldsbuilder.php i added in around line 177 inside "private function BuildTextField" just before ($result = '<div style="clear:both;">' .) the following code:
$autofillvalue=JRequest::getVar('autofillvalue');
if($autofillvalue) {
list($fieldname, $fieldvalue) = split('_', $autofillvalue);
if ($field['Name'] == $fieldname) { $value=$fieldvalue; }
}
now if you link to your form, lets say by this url:
www.myweb.com/contactform?autofillvalue=Subject_My subject text here
you get the field by name "Subject" (or any other fieldname you enter right after the "=") prefilled with the value you enter right after the underscore "_"
you can add this lines of code also inside "private function BuildTextareaField" just before ($result = '<div style="clear:both;">' .) to autofill a text area field.
the question which remains for me, what i didnt understand while searching for an answer in this forum is:
why does a field i create and name "Subject" gets stripped from the emails? it seems like it is a non-usable / forbidden name for a form field??? any fix on this?
. Use the ideas in this thread and this one with multiple name=value tags. 
