You can save the track as a line in Tracker Hunter 6 by exporting and importing it back as a GPX file to the App. The line will be stored permanently on the map until you removed it.
How to proceed:
On Tracker App> ”All Targets”> Select the target of which you want to save the traveled track> On the right top corner (three dots)>Select ”Share”> Choose “Share as a GPX file” > Select the start and end date > Click Ok. Then you can share the file via for example WhatsApp.
To import back> just open the GPX file and you should be offered the option to open it with TH6> Save as line or as area (disable area if you want to save as line)> Go to the App main menu> All Targets> My Markers > and scroll down you will find the file string > Click > you will see the line saved on map.