Enter DFU mode using open USB bootloader example on pca10056

Question - How to enter in DFU mode using open USB bootloader example on pca10056 ?

i tried writing nRF52840DK with examples-DFU-open_botloader-pca10056_usb and usb_debug by SWD debugging port J2 and when i plugged the usb to J3 connector and pressed reset key, nRF52840 is not being recognized by nRF connect application.