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 the new design; FIG. 2 is a front view of a second embodiment thereof; FIG. 3 is a front view of a third embodiment thereof; FIG. 4 is a front view of a fourth embodiment thereof; FIG. 5 is a front view of a fifth embodiment thereof; and, FIG. 6 is a front view of a sixth embodiment thereof. The outermost broken line rectangle illustrates a display screen or portion thereof and forms no part of the claimed design. The remaining broken lines illustrate portions of the graphical user interface that form no part of the claimed design.