let us suppose this is ur button in the BODY
<INPUT TYPE="BUTTON" VALUE="Testing" ONCLICK="javascript
:btnTesting_Click();">
and create an extra empty DIV in the place where you want the textbox.
<DIV ID="testDiv"></DIV>
Now write the btnTesting_Click() in the HEAD
<SCRIPT TYPE="text/javascript">
function btnTesting_Click()
{
document.getElementById('testDiv').innerHTML = "<INPUT TYPE='TEXTBOX' NAME='txt'>"
}
</SCRIPT>
Example
<HTML>
<HEAD>
<SCRIPT TYPE="text/javascript">
function btnTesting_Click()
{
document.getElementById('testDiv').innerHTML = "<INPUT TYPE='TEXTBOX' NAME='txt'>";
}
</SCRIPT>
</HEAD>
<BODY>
<INPUT TYPE="BUTTON" VALUE="Testing" ONCLICK="javascript
:btnTesting_Click();">
<DIV ID="testDiv"></DIV>
</BODY>
</HTML>