Go to settings - app lock - settings (top right) - hide content, also accessible from security app.
In order to control the app's notification you must first turn on the app lock feature first for it, located at the settings - applock.
Now that we have selected and locked our apps we can turn on the feature by going into settings (top right icon) - turn on "hide content" & we are done.
As you would notice as the notifications will come along for locked app, content will be shown as "hidden" over both the place at notification shade & over lockscreen (if you have enabled).
Check the original guide if you want illustrations/pictures on how to do it.