Subscription API do not redirect after payment confirmation

[BigBug]

Trying to implement "create subscription" by this documentation:

https://developer.paypal.com/docs/api/subscriptions/v1/#subscriptions_create

My POST looks like this:

{#336 ▼ // 
+"plan_id": "P-2D051204FR853833DMZG465Y"
+"custom_id": "customer_123"
+"user_action": {#339 ▼
+"return_url": "https://subscription-success"
+"cancel_url": "https://subscription-index"
}
}

But when user confirms subscription, he stays on PayPal page. I'm not sure, what is wrong here.

Thank you,