send value from client side to server side using C# in asp.net

I'm trying to create a website content management using asp.net and C# and bootstrap. I already done this using asp.net and C# and a server control like gridview but I want create this version one like as wordpress CMS.
I will describe my project to clear my purpose.
First I fill a DataTable from database. This Datatable has messageId int, Subject varchar, name varchar, email varchar, message text, isRead bit, and so on columns.isRead column is bit type for specifies that the message is read or not. I Fill my DataTable using below Method:

DataTable dt = cls.Fill_In_DataTable("MessageFetchMessage");

Then I generate html text using another method dynamically: on Page_Load

protected void Page_Load(object sender, EventArgs e)
{
   messeges = cls.fetchMessages();
}

messege the string variable, will append generated html code to aspx page:

<div class="row">
<div class="col-lg-12">
    <div class="panel panel-default">
        <div class="panel-heading">
            <h3 class="panel-title"><i class="fa fa-clock-o fa-fw"></i> Last messages From users</h3>
        </div>
        <div class="panel-body">
            <div class="list-group">
                <%=messeges %>
            </div>
            <div class="text-right">
                <a href="#">View All messages <i class="fa fa-arrow-circle-right"></i></a>
            </div>
        </div>
    </div>
</div>

the message content has these text from fetchMessages()method:

public string fetchMessages()
{
    string post = ""; string readed = "";
    DataTable dt = cls.Fill_In_DataTable("MessageFetchMessage");
    if (dt.Rows.Count>0)
    {
        for (int i = 0; i < dt.Rows.Count; i++)
        {
            DateTime dtTemp = DateTime.Parse(dt.Rows[i]["messageDate"].ToString());

            if (dt.Rows[i]["isRead"].ToString() == "True")
                readed = "MessageReaded"; 
            else
                readed = "MessageNew";
            post += "<div class='modal fade' id='myModal" + dt.Rows[i]["messageId"].ToString() + "' tabindex='-1' role='dialog' aria-labelledby='myModalLabel'>"
                + "<div class='modal-dialog' role='document'>"
                + "<div class='modal-content'>"
                + "<div class='modal-header'><button type='button' class='close' data-dismiss='modal' aria-label='Close'><span aria-hidden='true'>&times;</span></button><h4 class='modal-title' id='myModalLabel'><span style='font-weight:bold'>Subject</span> : " + dt.Rows[i]["subject"].ToString() + "</h4></div>"
                + "<div class='modal-header'><p><span style='font-weight:bold'>Date</span> : " + dtTemp.ToString("yyyy/MM/dd") + "</p>"
                + "<p><span style='font-weight:bold'>Time</span> : " + dt.Rows[i]["messageTime"].ToString() + "</p>"
                + "<p><span style='font-weight:bold'>Email</span> : " + dt.Rows[i]["email"].ToString() + "</p></div>"
                + "<div class='modal-body'>" + dt.Rows[i]["message"].ToString() + "</div>"
                + "<div class='modal-footer'><button type='button' class='btn btn-default' data-dismiss='modal'>Close</button><input type='submit' ID='btn" + dt.Rows[i]["messageId"].ToString() + "'  class='btn btn-danger' onserverclick='btn_Click' value='Delete message' /></div>"    
                + "</div></div></div>";
            string narrow = Special.TimeToNarrow(dt.Rows[i]["messageDate"].ToString(), dt.Rows[i]["messageTime"].ToString());
            post += "<a data-toggle='modal' data-target='#myModal" + dt.Rows[i]["messageId"].ToString() + "' href='#' class='list-group-item " + readed + "'><span class='badge'>" + narrow + "</span><i class='fa fa-fw fa-comment'></i> <span>"
                 + dt.Rows[i]["name"].ToString() + "</span> : <span>" + dt.Rows[i]["subject"].ToString() + "</span></a>";
        }
    }

    return post;
}

finally I add server code behind for btn_Click Event:

protected void btn_Click(object sender, EventArgs e)
{
    string id = (sender as Control).ClientID;
    //Then give Id to database class for CRUD Intractions
}

but btn_Click never called from client side. I search for similar question within 2 days and didn't get answer. please help me :)
Here I will put my Website screen Shots:

Then after click on one of the rows a pop up window will show using modal bootstrap: