You have to go through the azure portal and select your API
management component then click on the Publisher Portal in the
left top corner.
Go through the APIs and then Products tab.
Click on the ADD API TO PRODUCTS button and then select an
appropriate option.
Go through the azure portal again, but click on the Developer
Portal in the left top corner.
Click on the ComboBox which is placed on the right side and
choose Profile.
You will be able to see the access keys for the products and pick
the key for the product which you have chosen in the previous steps
(Starter or Unlimited).