Arthur, Marigold Knight
Haste Whenever Arthur and at least one other creature attack, look at the top six cards of your library. You may put a creature card from among them onto the battlefield tapped and attacking. Put the rest on the bottom of your library in a random order. Return that creature to its owner's hand at end of combat.
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Official rulings (2) · Scryfall
Although the creature you put onto the battlefield is attacking, it was never declared as an attacking creature. Abilities that trigger whenever a creature attacks won't trigger when that creature enters attacking.
You choose the player, planeswalker, or battle the creature you put onto the battlefield is attacking. It doesn't have to be the same player, planeswalker, or battle that Arthur or any other attacking creatures are attacking.