I claim the ornamental design for a sticker platform, as shown and described.
FIG. 1 is a front perspective view of my new design for a sticker platform;
FIG. 2 is a top elevational view thereof;
FIG. 3 is a bottom elevational view thereof;
FIG. 4 is a front elevational view thereof;
FIG. 5 is a rear elevational view thereof;
FIG. 6 is a right side elevational view thereof; and,
FIG. 7 is a left side elevational view thereof.
The broken line showing of stickers in FIG. 1 is included for the purpose of illustrating environmental structure and forms no part of the claimed design.