where do i find this location to start this objective

[diamond]

blind wells or wrathborn hunts

[Cheung K Huge]

dreaming city

[EckyThump]

When you spawn into The Dreaming City, you'll see a huge building with a large doorway. Blind Well is through that doorway, up some steps. You might enter an activity that is already in progress, or you might have to start it yourself and hope other players show up.

Wrathborn Hunts are Season 12's seasonal activity. You will need Season 12's pass to play it, so if you don't have that, then you'll need to do Blind Well instead.

[Snow Miser]

Question: Does it give you based on each wave completion of Blind Well, or are you required to run the ENTIRE thing?

[EckyThump]

Originally posted by Dr. Doofenshmirtz:

Question: Does it give you based on each wave completion of Blind Well, or are you required to run the ENTIRE thing?

Neither, technically. You get a set %ge completion (which differs per tier of Blind Well) when the activity finishes, no matter how much of it you actually participated in.
So someone arriving in the last moments of the boss fight will get the same amount as the guy who started the activity.