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 and Import 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: