Hi Paulo,
you have to use javascript. On the search button use something like
OnClientClick="javascript:OpenMyWindow(); return false;"

Now you have to define two javascript functions OpenMyWindow and UpdateValue
function.
function OpenMyWindow() {
 window.open("~/Customers.aspx", other params to set up your new window);
}

function UpdateValue(returnCustomer) {
 var customerTextBox = document.getElementById('customerTextBox');
 customerTextBox.Value = returnCustomer; // updates value in some html
textbox

 // you will probably have to refresh value here
 if (document.createEvent) {
   var onchangeEvent = document.createEvent('HTMLEvents');
   onchangeEvent.initEvent('change', true, true);
   customerTextBox.dispatchEvent(onchangeEvent);
 }
 else {
   customerTextBox.fireEvent('onchange');
 }
}

Customers.aspx will handle your grid with customers. You also need to
implement return mechanism on this page. Say for example if user select row
in grid it will return customer name back to previous window. You again need
to use javascript:

function Done(returnValue) {
 window.opener.UpdateValue(returnValue); // calls UpdateValue defined in
previous window
 window.close();
}

I don't say it will work exactly as I have described but at least it gives
you an idea.

Regards,
Ladislav