Hello,
Here are the steps to set a field value,
1. Click on "fx" icon in Action window.
2. Click on "Insert Field or Group" button in Insert Formula window.
3. Select secondary data source setting value and click on ‘Filter Data' button.
4. Click on ‘Add' button in Filter Data window.
5. Select filter condition field in left side dropdown and second drop down value should be ‘equal to'.
6. In third drop down select main data source selected node (filter on this field).
Click on ‘OK' seven times