Display Screen or Portion Thereof with a Graphical User Interface

FIG. 1 is a front perspective view of a handheld computing device with a display screen or portion thereof with a graphical user interface according to a first embodiment showing a first image in a sequence according to the new design; FIG. 2 is a front view thereof; FIG. 3 is a front view of a second image thereof; FIG. 4 is a front view of a third image thereof; and, FIG. 5 is a front view of a fourth image thereof. The design sequentially transitions between the images shown in FIGS. 2 - 5 . The process or period in which one image transitions to another image forms no part of the claimed design. Broken lines in FIGS. 1 - 5 show portions of a computing device with the display screen or portion thereof with a graphical user interface and form no part of the claimed design.