[MeRcyLeZZ]

#include <amxmodx>

new const MESSAGE[] = "Type your message here"

public plugin_init()
{
    //...
}

public client_putinserver(id)
{
    set_task(10.0, "show_hud", id)
}

public show_hud(id)
{
    if (!is_user_connected(id)) // player disconnected
        return;

    set_hudmessage(200, 100, 0, 0.02, 0.9, 0, 0.0, 5.1, 0.0, 0.0, -1)
    show_hudmessage(id, MESSAGE)

    set_task(5.0 , "show_hud" , id)
}

Check set_hudmessage to see how to change effect type, color, etc.