Display Screen or Portion Thereof Having a Graphical User Interface with a Splash Screen Icon

The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. FIG. 1 is a first embodiment of a display screen or portion thereof having a graphical user interface with a splash screen icon showing our new design; and, FIG. 2 is a second embodiment thereof. The broken lines showing an electronic device (the rectangular box surrounding the graphical user interface) illustrate environmental subject matter, whereas the remaining broken lines showing elements of the graphical user interface illustrate portions of the article. None of the broken lines form part of the claimed design.