Hi there...
In the SMTP Host you need to put your companydomain-com.mail.protection.outlook.com address
Then follow this ;
Option2 is the bit you are interested in..
So, you need to add the connector - from Your Organization to your Office 365 and put the external IP address of your server in (use ipchicken.com to find it)
Then update your SPF record in your Domain Control panel to add ip4:xxx.xxx.xxx.xxx
That should work...