Run the following test. I hope everything will then become clear. Both CFM pages are within the same directory.
testFormpage.cfm
<SCRIPT>
function fnselectcat()
{ document.frm.txt_expiration.value = 'KARAMBA!';
document.frm.submit();
}
</SCRIPT>
<body onload="fnselectcat()" >
<form name="frm" method="post" action="testActionpage.cfm">
<input name="txt_expiration" type="text">
</form>
</body>
testActionpage.cfm
<cfif isdefined("form.txt_expiration")>
The form has been submitted and the value of form.txt_expiration is: <cfoutput>#form.txt_expiration#</cfoutput>
</cfif>