4. Hold Touchpad/Change View To Chat
Weirdly, the custom chat prompt seems to be showing for some players and not others, but there is a way to deliver a typed out message to your squad, rather than relying entirely on the R1/RB Ping system.
Done by holding the touchpad on PS4 or Change View on Xbox One, this will bring up the console's keyboard so you can deliver more accurate messages or instructions to your teammates.
It must be said that what Respawn have achieved with the Ping system feels pretty progressive in terms of letting complex multiplayer interactions happen without the need to talk to strangers, but every now and then you just need to spell out "THEY'RE UNDER THE FLOOR" for all to see.