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; FIG. 2 is a front view thereof showing a second image in the sequence; FIG. 3 is a front view thereof showing a third image in the sequence; FIG. 4 is a front view thereof showing a fourth image in the sequence; and, FIG. 5 is a front view thereof showing a fifth image in the sequence. In these drawings, the outermost rectangular broken line in the figures illustrates a display screen or portion thereof and forms no part of the claimed design. The remaining broken lines represent elements of the graphical user interface that form no part of the claimed design. The stippling depicts a contrast in appearance. The appearance of the figures sequentially transitions between FIGS. 1 - 5 . The process or period in which one image transitions to another forms no part of the claimed design.