Display Screen, or Portion Thereof, with a Transitional Graphical User Interface

FIG. 1 is a first image in a sequence depicting a display screen, or portion thereof, with a transitional graphical user interface showing the new design; and,

FIG. 2 is a second image thereof.

The outermost broken line rectangle in the figures illustrates the display screen or portion thereof and forms no part of the claimed design. The remaining broken lines showing in the figures illustrates portions of the transitional graphical user interface that form no part of the claimed design. The appearance of the transitional graphical user interface sequentially transitions between the images shown in FIGS. 1 - 2 . The process or period in which one image transitions to another image forms no part of the claimed design.