Display Screen or Portion Thereof with a Graphical User Interface

FIG. 1 is a front view of a first embodiment of a display screen or portion thereof with a graphical user interface showing our new design; FIG. 2 is a second image thereof; FIG. 3 is a third image thereof; FIG. 4 is a fourth image thereof; FIG. 5 is a fifth image thereof; and, FIG. 6 is a sixth image thereof. The outermost dash-dot broken lines in the drawings illustrate the display screen or portion thereof and form no part of the claimed design. The dotted broken lines in the drawings illustrate portions of the graphical user interface that form no part of the claimed design. The short dashed lines extending to and from the vehicle icon in FIGS. 3 - 6 are part of the claimed subject matter. The appearance of the transitional image transitions between the images shown in FIGS. 1 - 6 respectively. The process or period in which one image transitions to another forms no part of the claimed design.