Sending a text message to a shortcode number

We're trying to sign up for a notification service and need to send a message FLCOVID19 to 888777 - but our Google Pixel 2 phone will not 'send' it.

I cannot find a setting on the phone and the service, operated by www.nixle.com, is legitimate.

Not sure whether this is a phone settings problem or a phone service issue (Xfinity) as some services apparently need to allow 'shortcodes'.

Advice welcomed!