implement funtion result in asp:textbox

Hi,
I have this code:
<asp:textbox runat="server" textmode="MultiLine" id="content" name="content" onkeydown="setTimeout('addZeile(document.getElementById(\'content\'))',10)" style="font-size:14px; width: 750px; height: 200px;" cols="97" rows="10" /> and would like to add a the text tag to it which would be filled in with the result of a function. In the version I had before (html textarea) it was simply implemented with <%=proofedanz()%> between the textarea tags but this does will not work as the filled in text would come within the tag shown above like text="hi" but instead of 'hi' I would like to insert proofedanz(), which returns the result of that function.
How do I code this?

Thanks in advancem
Pascal