Correct phrase for this kind of parking lot sign
Solution 1:
I found one example from the San Diego Union Tribune regarding backing into on-street parking:
But this uses back-in as a compound adjective, which is not particularly good.
The UK has standardised road signs, and this directive isn't included, so signs in car parks are generally custom-made, using words like Reverse into this space. But in my experience they are generally directions not to reverse into the space.
Solution 2:
The google searches at these links show many example English signs... reverse parking only signs and back in parking only signs