Which event will trigger?

I would do it in FORM_OnAfterTransaction:

// was the transaction successful?
if (formState.agsError) return;

// following an inquire transaction, set the field
var fc=data.getElementsByTagName("_f1");
if (fc && fc.length > 0)
fc=fc[0];
if (fc == "I")
lawForm.setFormValue("xxxx","yyy");
}

But I need it before a transaction. Somewhere after the form is displayed, but before data entry starts.

It needs to be displayed prior to them entering data. Basically to work the same as if you had "defaulted" the value in Design Studio for the field. They are not doing an inquire prior to entering data.

Our reason is we have site-based purchasing (every user is a Buyer). The location is their site, so we want to display that has the default, but still allow some users to change if needed. I do have it working with the on blur, but I'm not sure thats the best way. Seems there should be SOME event that would allow you to perform action prior to a user doing anything. I though the onInit would at first, but it just gets cleared out.

Try FORM_OnAfterDataInit...there's an example in the DOM viewer much like what you want to do:

function FORM_OnAfterDataInit()
{
lawForm.setFormValue("text1", portalObj.getUserVariable("VENDOR_GROUP"));
var venGrp = lawForm.getFormElement("text1");
if (venGrp)
vengrp.disabled = true;
}