How to set a default value in custom link field

in the sales order list am getting three values as drop down, i need to set the first value as default value.

add another onload event to the form

frappe.ui.form.on("Opportunity", "onload", function(frm) {{....
    callback: function() { frm.set_value(...) };

Need some more light on this, How to use set_value to set first value of list?

Eg, I used custom below script to filter broker_address field, not how to set first value of this as default.

cur_frm.set_query("broker_address", function() {
	if( {
		return {
			query: "frappe.geo.doctype.address.address.address_query",
			filters: { link_doctype: "Supplier", link_name: } 
	else frappe.throw(__("Please set Broker"));