Get a sturdy, thin metal bar (say 4mm x 15mm cross section) from the hardware shop.
Have the bar bent into an 'L' shape, with one arm about the length from the floor to the cupboard's handle. (Black figure in the diagram.) Loop a piece of string (pink figure) around the top of the 'L' to help guide the rod later.
Work the door and cupboard so that you can move the bar past the door through the 'L' gap as shown in the diagram, leaving part of the rod sticking out the bottom on your side of the door. Use this part as a handle to manipulate the bar.
Close the door fully, hanging on to the string at the top and the bar's handle at the bottom. Work the handle while pulling on the string until the cupboard door is completely closed. The handle would tend to push the bar to the left and away from you, while the string would tend to pull the bar towards you (and to the left).
Open the door.