Display Screen or Portion Thereof with a Transitional Graphical User Interface as Applied to a Computer-based Game

FIG. 1 is a front view of a display screen or portion thereof with a transitional graphical user interface as applied to a computer-based game showing a first image in a sequence according to the new design; and,

FIG. 2 is a front view thereof showing a second image in the sequence.

The outermost broken line illustrates a display screen or portion thereof and form no part of the claimed design. The remaining broken lines represent elements of the display screen or portion thereof with a transitional graphical user interface that form no part of the claimed design.