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Background
There are two places where you can trigger this quest: at the border post between Velen and Novigrad (#1) or at the gate leading to Oxenfurt (#2).
Walkthrough
When you try to access an area where you're not allowed, a Redanian soldier will stop you and tell you that you'll need a pass in order to go through. The soldier at Oxenfurt won't have any helpful advice for you, but the soldiers at the border post will hint that you can get a pass through the black market.
There are three ways to get a pass:
When you return to the soldiers with one of these passes, they'll open up the gates to you, and you'll earn 75 xp.
1 - Border Post
2 - Captain of the Guard
3 - Shady Merchant