Assign to a HTML textbox a value with asp.net

I had to create a HTML textbox to add the date time but I don't know how to assign the value with asp.net, how can I do that?

This is my HTM Textbox code:

<input id="txtFechaPlan" name="txtFechaPlan" style="width: 124px; position: static" type="text" />

Thx.
...