Signing in to Microsoft Office 365/Copilot is a slightly different process from many other Microsoft apps.
Opening 365 will prompt a sign-in page as shown below.
Clicking the button will open a separate sign-in window. If you do not see your account under the "use this account" section, click on "Work or school account" and then "continue".
If you do see your account on this page, simply select it and press "Continue" and you should be signed in automatically.
In the next page, click on "sign in options," then on "sign in with an organization."
Enter "lyndendoor.com" (no quote marks) and hit next.
You should be taken to the standard Lynden door SSO sign in screen. You can enter your Lynden Door email (should look like jSmith@lyndendoor.com) and your current computer password.