Display Screen or Portion Thereof with a Graphical User Interface

FIG. 1 is a front view of a display screen or portion thereof with a graphical user interface showing my new design in a first embodiment;

FIG. 2 is a front view of a display screen or portion thereof with a graphical user interface showing my new design in a second embodiment; and,

FIG. 3 is a front view of a display screen or portion thereof with a graphical user interface showing my new design in a third embodiment.

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 and form no part of the claimed design.