Change value of text box on Mouse Up

newbie to javascript. I have a form with many checkboxes that need to be 4-cycle (Yes, No, Not Applicable, Blank) rather than 2 cycle so I went with a text box rather than a check box.

Using a button and a text box, clicking the button cycles through all 4 values, placing them in the text box whose font is set to Wingdings2.  I would rather use just the text box and a Mouse Up action but I don't know how to write that code.

This is what works for the button/text box combo:

var t = this.getField("TextTest").value;

switch(t) {

case "":
this.getField("TextTest").value = "P";
break;

case "P":
this.getField("TextTest").value = "O";
break;

case "O":
this.getField("TextTest").value = "X";
break;

case "X":
this.getField("TextTest").value = "";

}

How can I do this with just the text box?  I would appreciate any help on this.  Also please let me know if I am setting myself up for future headaches doing it this way.

Thanks

Shaun