You cannot do this with HTML. However, you can include a script after the form that sets the focus to the appropriate field, like this:
<form id="myform" name="myform" action=...>
<input type="text" id="myinput" name="myinput" ...>
A similar approach uses <body onload=...> to set the focus, but some browsers seem to process the ONLOAD event before the entire document (i.e., the part with the form) has been loaded.