Go to the settings menu (symbol with 3 dashes) and click on 'My wholesalers'.
Then select the market in which the wholesaler is active. You will now see a list of wholesalers. Wholesalers to which you are already connected are listed on 'Active'.
Click 'Add' if you want to order from this wholesaler. In the next screen, you will see a confirmation that you are going to make a request to the wholesaler in question. Click on 'request'. As soon as this wholesaler approves your request, it will automatically appear in the App under 'Active' and you can start ordering.