The ornamental design for a displayscreen or portion thereof with a graphicaluserinterface for an on-board device, as shown and described.
The FIGURE is a front view of a display screen or portion thereof with a graphical user interface for an on-board device, showing our new design.
The outermost rectangle in broken lines shows a display screen or a portion thereof and forms no part of the claimed design. The remaining broken lines illustrate portions of a graphical user interface and form no part of the claimed design.