Hi manionasp,

Refer the below article. It working with Mouse hover selection and  keyboard down arrow selection.

Get Selected Item Value from jQuery Autocomplete TextBox in ASP.Net using C# and VB.Net

C#

protected void Submit(object sender, EventArgs e)
{
    string customerName = Request.Form[txtSearch.UniqueID];
    string customerId = Request.Form[hfCustomerId.UniqueID];
    ClientScript.RegisterClientScriptBlock(this.GetType(), "", "alert('Name : " + customerName + "\\nId : " + customerId + "')", true);
}

VB.Net

Protected Sub Submit(ByVal sender As Object, ByVal e As EventArgs)
    Dim customerName As String = Request.Form(txtSearch.UniqueID)
    Dim customerId As String = Request.Form(hfCustomerId.UniqueID)
    ClientScript.RegisterClientScriptBlock(Me.[GetType](), "", "alert('Name : " & customerName & "\nId : " & customerId & "')", True)
End Sub

Screenshot