In your GA Property Settings, click on Data Streams(BETA). Select the correct web data stream, and click on the edit icon of ‘Enhanced measurement’. On the right-hand panel, you can see all the enhanced measurement settings. Make sure the toggle for ‘Site search’ is on. Click on the cog – ‘Show advanced settings’, update ‘Search Term Query Parameter’ and ‘Additional Query Parameters’ fields and save changes.
Unfortunately, this is currently not available in App + Web.