How can I add a new value to a multi-value field programmatically?
Using this:

$account-&gt;get('field_useragent')-&gt;appendItem($_SERVER['HTTP_USER_AGENT']);


However, that doesn't seem like a good idea to me. Over time, that list will grow big, multi-value fields don't scale that well. All values are always loaded, on save, all values are inserted again into the database.

Maybe you want a list of unique user agents, but even that is hard to control with versions and so on. If you want a log, then save it somewhere else in your own table.

http://wizzlern.nl/sites/wizzlern.nl/files/artikel/drupal-content-entity-8.0.pdf doesn't have this yet, would probably make sense to add it.