demo： 

function FCKeditor_OnComplete( editorInstance ) 
{ 
editorInstance.EditorDocument.attachEvent("onkeydown", editor_keydown); 
editorInstance.EditorDocument.attachEvent("onkeyup", editor_keyup); 
} 
function editor_keydown() 
{ 
var oEditor = FCKeditorAPI.GetInstance('inputmsg') ; 
var keycode = oEditor.EditorWindow.event.keyCode; 
if(keycode==13) { 

} 

} 
function clearInput() 
{ 
var oEditor = FCKeditorAPI.GetInstance('inputmsg') ; 
oEditor.Commands.GetCommand( "NewPage" ).Execute(); // oEditor.SetHTML("") ; 
} 

function editor_keyup() 
{ 
var oEditor = FCKeditorAPI.GetInstance('inputmsg') ; 
var keycode = oEditor.EditorWindow.event.keyCode; 

if(keycode==13) { 
clearInput(); 
} 
}