Here's how you can add a new Send-as address or email alias to Mixmax:
Go to Settings in Gmail (Click the gear icon in the top right corner and select See all settings from the dropdown).
Go to the Accounts tab.
Next to 'Send mail as:' click 'Add another email address' and enter the email address.
Click Import Email Addresses to Mixmax — see the screenshot below:
Once you import your email addresses to Mixmax, you will be able to select the new email address in the From field of the Compose window:
Sending an email using an alias with Outlook is not currently supported by Mixmax.