How to send an email on behalf of Someone using Mac Mail

Add your manager's email account to the Mail app.

Add or remove email accounts in Mail on Mac - Apple Support

If you are talking about email relaying that is a different problem. Many email servers do not allow relaying (sending an email from a domain using another domain's address).

If you are both using the same email server/domain then just set the default from field to your manager's email address.

Just so you know, Mac Mail is an email app which is installed on all Macs with their operating system. Outlook is an email app which can be added if desired. It is apparently more designed for business situations. To use Mac Mail, you will have to follow the instructions to set up an account so it can connect with the email server in order to send or receive emails. You can also check to see if Mail can import settings, etc. from Outlook. (I don't use Mail myself, so I can't say for sure.) Also, when using Mail, it works with the Contacts app. The Contacts app is where your email contacts will be stored. Contacts does have the ability to import email contacts from other email apps. I am certain that Outlook also has the ability to export them. Not all apps offer exactly the same formats for importing and exporting data, so you have to determine one format that is common to both apps. If you think this is all somewhat difficult - it is. So, no one can tell you precisely what to do. You will have to read and understand all the instructions which are provided in each app. I strongly recommend that you take copious notes. I speak from experience with this issue.