You can give any input a default value.  Our current form is a log in form so you would not want to put the data in for the user.  However, you can change the value of buttons as well.  The way to give a text field an automatically completed value is the same as shown below for the submit button.

Currently your submit button code looks like this:

<input type="submit">

change it so it looks like this:

<input type="submit" value="Send this info">

Try the page again and the button should have new text on it.  If you do that for an input field the user can still change it if they want to.