How to lock habitation door from the inside

Inside habitation door is a very  sturdy looking lock mechanism. As  new owners of this Midas we have no idea how to lock the door from the inside.
Advice please.

When you've opened it from outside return key to vertical position, you should then be able to lock from inside by turning the knob to one side. It should remain in that position and not spring back.

Would be nice if you'd say hello as requested.
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On my nuevo you push the inside opening latch inward towards the door .