curl --request GET \
  --url https://api.hubapi.com/marketing/v3/campaigns/{campaignGuid}/budget/{budgetId} \
  --header 'Authorization: Bearer <token>'

{
  "amount": 123,
  "createdAt": 123,
  "id": "<string>",
  "name": "<string>",
  "order": 123,
  "updatedAt": 123,
  "description": "<string>"
}