Augur Raid is a random event, and you can't manually trigger it.
*** Spoiler - click to reveal ***Keep running expeditions until you see a message Giant bubbles are forming.
A bubble will spawn from the sky, walk into the bubble on the ground, that will start the raid. The bubble mark will also be displayed in the map.
Once inside the dungeon, defeat the boss and you will see the message The master of bubbles was felled.