Problem with getting field's value in Page

I am having trouble fetching the value of a link field (custom) which i created on a page. What I want is whenever any value is selected in link field, I get that value on change.

I am having a .js and .py file for a page.
I am writing a sql query where the value selected in link field of page will go to my query in where condition But I’m having trouble in doing so. Here’s my code


frappe.pages['customer'].on_page_load = function(wrapper) {
        var page = frappe.ui.make_app_page({
                parent: wrapper,
                title: 'Customer',
                single_column: true

        frappe.show_alert('Hey '+frappe.session.user,3)

        page.custom_field = page.add_field({
                fieldname: 'customer',
                label: __('Customer'),
                fieldtype: 'Link',
                options: 'Customer',



        method: '',
        args: {},


import frappe


def details(name):
     customer = frappe.db.sql("""select customer_name,lead_name,email_id,mobile_no,primary_address from `tabCustomer` where name = %s""",name)
     return {'Customer Name':customer[0][0],'Lead Id':customer[0][1],'Email':customer[0][2],'Mobile':customer[0][3],'Address':customer[0][4]}

I want that the name of customer selected in page, when selected I get all the data of that customer,
Any suggestions ?