Ver originalfunction f(){if(document.frm.sel.options.value == "el_value_del_option_que_quieras"){document.frm.area.value = "hola que tal";return true;}else return false;}
Ver original<form name="frm"><select name="sel" onChange="return f()"><option value="1">Opción 1</option><option value="2">Opción 2</option><option value="3">Opción 3</option></select><textarea name="area"></textarea></form>