Forum Post: RE: How to determine, on form load, the entity type for a field, then use that field to set another read-only field.

I think you can simplify it down to this:

function setQuoteFor() {     var lookup = Xrm.Page.getAttribute("customerid").getValue();     if (lookup != null) {         if (lookup[0].entityType == "contact") {             Xrm.Page.getAttribute("new_quoteforentity").setValue(100000000);         }         if (lookup[0].entityType == "account") {             Xrm.Page.getAttribute("new_quoteforentity").setValue(100000001);         }     } } 

A couple things to note:

JavaScript is case sensitive so XRM is not the same as Xrm  - Xrm is correct

Field names - are all lower case - on the screen showing the field data under customizations - use the Name field

Option set fields can be set with just the integer value - since it is a number there won't be any commas

You started out referencing "lookupItem" and then went to "lookup"

But you are on the right track :)