Maybe a double-tap on a non-map area of the screen?

Like the directions area, top left, which is an easy area to aim for during a ride/drive.

A nice bright confirmation popup saying 'Locked' could let the user know the double-tap has worked.

It would also be wise to add a new Setting option to actively enable it just in case innocent new users starting double-tapping randomly, and lock the app.

@Con-Hennekens

That's great, thanks for that. Although, in my defence"Place POI" is not that obvious what the cross actually does.