Webhook question

I have a brief question about webhooks.  I have a webhook that creates a record in one table when a record is created in the source table.  Would I use the field ID for the source table or destination table?

This is the body of the webhook

<records_csv>
<![CDATA[
%RepeatOn%
[Project Number]
%RepeatOff%
]]>
</records_csv>
<clist>485</clist>
</qdbapi>

I am getting a 400 error at the moment.