prepopulate form from a hook within wordpress function.php file

I have a form that I created by simply using the HTML editor in the wp-admin backend for a page. I have some variables in a hook for that page, I would like to pre-populate some of the form fields with the variables in the hook.

add_action('wp', 'simple_form');

function simple_form(){
    $firstname = 'Bob';
    //Do something to populate the form field name <fname> with "Bob"
}