static Text: myTextDraw = INVALID_TEXT_DRAW;

public OnGameModeInit()
{
     myTextDraw = TextDrawCreate("_", 340, 210);
     return 1;
}

COMMAND:mycommand(playerid)
{
     // Use any of the functions from https://sampwiki.blast.hk/wiki/Category:Textdraw to manipulate the textdraw
     // the ID of the textdraw is stored in the variable 'myTextDraw'
     // Example 
     TextDrawSetString(myTextDraw, "New!");
     // Make sure you re-show it for the changes to take effect
     return 1;
}