curl -X POST "https://api.usegately.com/api/v1/plans/plan_123/subscribe" \
-H "Authorization: Bearer YOUR_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"success_url": "https://mysite.com/success",
"cancel_url": "https://mysite.com/pricing",
"customer_email": "user@example.com",
"user_id": "user_123"
}'