Fills the polarion-lookup-method property.

def _fill_lookup_prop(self, requirements_properties):
        """Fills the polarion-lookup-method property."""
        if not self._lookup_prop:
            raise Dump2PolarionException("Failed to set the 'polarion-lookup-method' property")

        etree.SubElement(
            requirements_properties,
            "property",
            {"name": "lookup-method", "value": self._lookup_prop},
        )