asp:button on popup not clicked

which jquery code ???popup or ajax post

my html code in master page is

login

on the above click, a pop up box with below html code

<input class="loginName required" type="text" id="loginname" value="Username" name="loginName" />
<input class="loginPass required" type="password" id="loginpassword" value="Password" name="loginPass" />

<input type="submit" value="LOG IN" id="LoginBtnValue"/>

on login click

function GetLoginDetails(UserName,UserPassword)
{
//alert(UserName+UserPassword);
var returnurl = window.location.href;

$.ajax({
type: "POST",
contentType: "application/json; charset=utf-8",
url: "Default.aspx/GetLoginDetails",
data: "{'UserName':'" +UserName+ "'," + "'UserPassword':'"+UserPassword +"','returnurl':'"+returnurl+"'}",
success: function(data) {
var theHtml = data.d;

if(data.d=="loginerror")
{
alert('Username or password is incorrect');
}
else
{
window.location.href = data.d;
}
},
error: function(xhr, textStatus, errorThrown) {
alert("Login FAIL: " + xhr + " " + textStatus + " " + errorThrown);
}
});

}

with values

and in default.aspx.cs ,
web method like

[WebMethod]
public static string GetLoginDetails(string UserName, string UserPassword,string returnurl)
{


}

In this i want to [WebMethod]
public static string GetLoginDetails(string UserName, string UserPassword,string returnurl)
{

}


what i want is to change master page text login to logoff with name (for exp, logoff , welcome name)

how to get master page control in content page web method