. Net Control page with js assignment background does not receive data Solution

I used
[Html]
<Asp: TextBox ID = "txtEndDate" name = "txtEndDate" ReadOnly = "true" runat = "server" onclick = "J. calendar. get (); "> </asp: TextBox>
The purpose is to click the text box to bring up a js date control. 1

(Figure 1)
After the date is selected, use the following background code to receive parameters.
[Csharp]
String sEndDate = txtEndDate. Text;

The value obtained from the result is an empty string ("").
My solution
① Add a name to the TextBox Control above and use the following code to solve the problem.
[Csharp] view plaincopyprint?
String end = Request. Form ["txtEndDate"];

Note: neither Request. QueryString ["txtEndDate"] can be obtained.

My current doubts
① Why cannot I receive the original attribute (Text attribute) for the value set in js)
② Why can't I get it only through Request. Form [""]?
③ There are no other solutions, so that normal properties can be used to receive