The ornamental design for an absorbent articlesubstrate with a pattern, as shown and described.
FIG. 1 is a front view of an absorbent article substrate with a pattern;
FIG. 2 is a rear view thereof;
FIG. 3 is a top end view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof; and,
FIG. 6 is a bottom end view thereof.
The broken lines, including those showing a diaper and diaper tabs and ears thereof, are included for the purpose of illustrating environment and form no part of the claimed design.