Join Distribution List in Microsoft 365

In Outlook on the web click the settings gear

Open

Start typing 'Distribution' in the search box, select 'Distribution groups'

Open

Under 'Distribution groups I belong to' click the join button

In the pop-up enter the name (or partial name) of the distribution list in the search box and hit enter, select the list you would like to join and then hit the join button

Click OK in the pop-up after successfully joining the list