By default, the send from name and email address is the account name and the specified account email address (see here on how this is set up)If you would like to change the send from name and email address on a per email send basis here's how to do it.
1. In your event menu, select Communications
2. If you're scheduling an email send to multiple contacts select Add Scheduled Email or if you're just sending a one off email to one contact select Individual Email.
The custom send from name and email address boxes will appear in either selection (the image below is from the Add Scheduled Email option).
Add your send from name and email address.
Important note time! If the email domain is different from the account email domain then you'll need to set up SPF and DKIM records to that domain to improve deliverability. There's an article here on how to do it, but if you need help please raise a support ticket via this site and we'll jump on it for you.
Once you're entered your send from name and email address, complete the email as usual and your chosen from name and address will appear in the recipient's inbox.