First, place the marker(s) anywhere on the map, and set the type to type to "Empty". Set the text to be whatever you want shown when the marker is visible.
To make it visible, run the code below in the Activation field of a trigger, or through a script, whatever:
For this example, a unit is named hGuy, and a marker is named markerNeedHelp.
"markerNeedHelp" setMarkerPos getpos hGuy;"markerNeedHelp" setMarkerType "WARNING"
AKAIK you cannot create a marker at runtime, you can only change the marker type. Placing a marker type of "Empty" in the editor makes it invisible at runtime...so changing it to some other type at runtime makes it appear.
Also, AKAIK you can't change the text of a marker, so you have to set the text in the editor and live with it throughout the mission.
The command reference has the marker type strings.
Note - when working with markers, the name is treated as a string so it has to been enclosed in quotes...unlike most things in the game which are treated as objects and don't need quotes.