Logging into Lightning for Outlook
|Knowledge Article Number||000231007|
|Resolution||Once you have enabled Lightning for Outlook in your Web App (OWA), Outlook 2016, or Outlook 2013, Click on the Salesforce tab loaded in the email window
and then click on the Log in to Salesforce blue button.
The App will prompt you for your Salesforce credentials, where you can provide your user details. As you can see below it will provide you with a new key that you must copy and paste back into the login page
* If clicking on the blue Log in to Salesforce button does not open the page you can apply the IE optimal settings article and try again
Reference the following video for more information: