If the store is placed on the Yandex.Market through an advertising agency, the default partner API access is open to employees of that agency. To work independently with the API, the shop can contact their agency. See more details in the Yandex.Market documentation.
NoteA single login can be provided access to a single ad account only. In other words, if your login already have the access to the ad account either on Yandex.Market or Yandex.Direct, then the agency can't share the access to another account with this login. You'll need to generate a new login to access a new account.