Exempting App in HTC Power Manager
Select Battery optimization
Open your Android Settings and click on the "Power" menu
Tap the "Battery optimization" menu item.
Tap on "Not optimized" at the top of the list to open the filters popup.
When the popup appears, select All Apps.
Select SMS Tracker Plus or SMS Tracker from the list.
When the popup appears, select Don't optimize and tap DONE.
Congratulations! You're all done.