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="20%"></asp:TextBox>
<a title="Pick Date from Calendar" onclick="calendarPicker('<% txtstart.ClientId %>');" href="javascript:void(0);" rel="nofollow noreferrer noopener" rel="nofollow noreferrer noopener">

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?

Answers:

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Method 1

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);" rel="nofollow noreferrer noopener" rel="nofollow noreferrer noopener">
</a>

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