You can set your default address in Gmail and then import it into 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:
Under Send mail as click make default next to the address that you want to be your default sending address.
Click Import Email Addresses to Mixmax — see screenshot below. (You will see this button only if you have downloaded and enabled the Mixmax extension in Gmail.)