Ashley should add this easy option to textbox.
Anyway, you can use calljs plugin.
First, add a textbox and set an <font color="red">ID_textbox</font> in the object's properties.
Then,
<font size="2">[i]When (action)[/i]
--> [b]CallJS[/b] - Execute [i]"var a=document.getElementById(""<font color="red">ID_textbox</font>""); a.blur();"[/i]</font>