Check one of two boxes based on field value

I am trying to accomplish (what I think would be) a pretty simple task, but cannot seem to get it to work. In short, I have one text field and two check boxes. The text field will be autopopulated from an external system with either M or F. Based on that field value, I would need either the Male or Female checkbox checked. I have tried the below, on the field value with On Focus as the trigger, but it does not do anything. I also tried it as a document level script, but that is not working either (it may be that the document level script executes before the field is autopopulated?)

var c1 = getField("Check Box1");
var c2 = getField("Check Box2"); if (this.getField.value = "m") {  c2.value = "On";}