Steam is the Windows storefront listed as a launch platform for Halo: Campaign Evolved’s Collector’s Edition voucher.
The edition details identify the voucher as a code for the Premium Edition upgrade content, distributed through the Steam version. That makes Steam the storefront answer, rather than Microsoft Store, GOG, or Epic Games Store.
The distinction matters because Halo: Campaign Evolved is also associated with Xbox and PlayStation platforms, while the question asks specifically about the Windows storefront. Steam is a PC distribution platform, not a separate console ecosystem, and the wording points to the storefront named in the edition listing.
A related detail is that the voucher includes a single code covering the base game and Premium Edition upgrade content. That is separate from preorder bonuses, which provide cosmetic items such as armor and weapon skins.