Actually, it’s not really possible:
The Wireless Emergency Alert system was established in a 2006 law, and became fully operational in 2012. The original law says that wireless carriers can allow their customers to block emergency alerts, except the Presidential ones.
The Presidential alerts are meant for situations of national importance: if a nuke were headed our way, for example, or some other serious, nationwide news. It’s not meant as an alternate outlet for petty, insulting tweets, and the message has to go through FEMA—it’s not sent directly from the President’s phone.
Signed by George W. Bush, went into effect under Barack Obama, and now being tested by the Trump administration’s FEMA. In theory, this is a good idea for true emergency situations, but, with this administration at the helm, who knows?