It might be a more suitable solution to store an attribute in your flow network to signify that it flows into said polygon. You could store the name of the National park for example, or Null when it doesn't intersect. eg: Field name called 'National Park Name'
(To identify the National Park attribute, just run a quick analysis - If line intersects national ...
The AutoTrace plugin can only be installed in versions 2.x of QGIS so you can not install it in a version 3.x installation.
The functionality of the AutoTrace plugin is now available on the Snapping toolbar (you will probably need to enable this).