How can I start an illager raid via commands?

The command is:

/summon pillager ~ ~ ~ minecraft:spawn_as_illager_captain

The three tildes (~) means it will be summoned at your exact location. you can change this by replacing it with coordinates with a space in between:

/summon pillager 100 65 -57 minecraft:spawn_as_illager_captain

Hope this helped!

To start a raid, use this command in the chat, and you will get the Bad Omen effect. To make a raid happen, just walk into a village in survival mode for it to start.

/effect @s bad_omen

Unfortunately, you have to be in a village to participate in a raid. Fortunately though, you can create your own village by only having 1 villager next to or near it's linked bed. You can check to see if it is linked to a bed by checking to see if it sleeps in said bed at night. Once you do this, simply use this command and the raid will start.

/effect @s bad_omen

Just note that if the villager is killed at any point between the start and end of the raid, the raid will be counted as a defeat.