Need the clientId of a textbox inside a content control using javascript

I have a masterpage that contains all the javascript and inside the content control, there is a link that calls a javascript function and I want to pass the id once it's rendered differently by the server.

<asp:TextBox ID="txtstart" runat="server" Width开发者_开发技巧="20%"></asp:TextBox>
<a title="Pick Date from Calendar" onclick="calendarPicker('<% txtstart.ClientId %>');" href="javascript:void(0);">

However, I keep getting this error:

Property access must assign to the property or use its value.

How would I be able to accomplish this?

Try this instead (notice the <%= in the onclick attribute):

<asp:TextBox ID="txtstart" runat="server" Width="20%" />
<a title="Pick Date from Calendar" 
   onclick="calendarPicker('<%= txtstart.ClientId %>');" 
   href="javascript:void(0);">
</a>