Yes. If you want to purchase an item from the Guild Wars Online Shop, you must have an ArenaNet account. If you already have one, simply log into your account when making your purchase. If you do not have an ArenaNet account, this article will assist you with creating one.
If you play Guild Wars Reforged on Steam, you will be able to purchase additional items and content from the game's Steam page under the "Content for this game" section. Please note that some items are not currently available in the Steam store but are expected to be added at a later date.