parameters in extra files

I'm submiting an iDoc request as an XML attached extra file. When I run the script all is OK but when I attempt for parameterise the XML extra file the script fails to recognise the parameters and sends them straight through as curly brackets.

For example I the XML file contains:

<POSTING_DATE>20160818140006</POSTING_DATE> which I change to

<POSTING_DATE>{date}</POSTING_DATE>

When I replay instead of sending 20160818140006 I send {date}

How do I send the value ?

jc