I found the solution and its called Attribute Value Templates (AVTs): A notation supported in Orbeon Forms
which makes it possible to use XPath in attributes. Simply put your Xpath within curly brackets as such:
<xforms:submission id="list-submission" serialization="none" method="get"
resource="{instance('main-instance')/data/some-element/@id}"
replace="instance" instance="additional-instance">
In my case, the resource url I wish to extract is found in the id attribute of the element "some-element" in my main instance.