How to submit ASP.NET forms with the enter key
Read up on the defaultbutton property for forms and panels. This should get you what you want. From the site above:
ASP.NET 2.0 introduces a wonderful work around for this. By simply specifying the "defaultbutton" property to the ID of the , whose event you want to fire, your job is done.
The defaultbutton property can be specified at the Form level in the form tag as well as at panel level in the definition tag. The form level setting is overridden when specified at the panel level, for those controls that are inside the panel.