How do I log into a Shared Mailbox that I have delegated rights to?

In the browser of your choice go to https://outlook.office.com/
Login using your company credentials
Once you are seeing the Outlook web view > click on your picture (or initials) in the upper git corner of the page
Then click the link "Open another mailbox"
In the following screen type in the email address of the shared mailbox you are trying to access
When the auto search presents the correct one click it
It will open the mailbox for you.

When you are in that shared mailbox you can use the tools like setting something to be blocked as spam, just as you would in your own mailbox.