The ornamental design for a displayscreen or portion thereof with a graphicaluserinterface, as shown and described.
FIG. 1 is a front view of a first embodiment of a display screen or portion thereof with a graphical user interface showing my new design; and,
FIG. 2 is a front view of a second embodiment of a display screen or portion thereof with a graphical user interface showing my new design.
The long irregularly spaced broken lines interspersed with dots are elements that form part of the claimed design. The shorter regularly spaced broken lines showing the display screen or portion thereof and portions of the graphical user interface form no part of the claimed design.