By default. When you have a button in a form and have clicked on an element of the form, pressing enter will submit the form. There really is nothing to be done.
If you want to force the button so its ready when the page loads, you can use a javascript onload event to give focus to the button immediately. Instead of having to click on an element of a form.
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Ignorance is not necessarily Bliss, case in point:
Unknown has caused an Unknown Error on Unknown and must be shutdown to prevent damage to Unknown.