Add Mailgun Delivery Server 

1) Login to your Mailgun console. If don't have an account, create one.
2) Navigate to Sending > Domains. Any domains that you wish to send emails from, should be added here. 

3) In Add domain page, enter the domain that you wish to use for email sending. Choose "US" as domain region. 

4) Add and verify the necessary "DNS records for sending" shown.

5) Navigate to Settings > API Keys > API security(tab). Copy and fill the "Private API Key" and "HTTP webhook singing key" in add delivery server page.