How can i change the form default value but not via hook_form_alter I have a question about add a node which have a specific field!

I create two buttons on different page called A,B.

When User touched the A button,i redirect the user to the node_form page.

The specific field default value will be set as ValueA.

When the user touched the B buttons,the value of the specific field will become ValueB.

So i cant user hook_node_form_alter.

I want to write the code at the myform_submit() because of touching the button.

How can i do that?

Are there any method which can change the form value not via the form_alter()?

Please Help Me fix this problem! Thank you!
Let assume you want to create a page. Then create button A which redirect user to /node/add/page?button=a and button B redirect to /node/add/page?button=b

Then in your module create hook_form_alter for node form

function MODULE_node_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form['#node']-&gt;type == 'page') {
    if ($_GET['button'] == 'a') {
      $form['FIELD_NAME']['und'][0]['#default_value'] = 'aaa';
    }
    elseif ($_GET['button'] == 'b') {
      $form['FIELD_NAME']['und'][0]['#default_value'] = 'bbb';
    }
    else {
      $form['FIELD_NAME']['und'][0]['#default_value'] = 'ccc';
    }
  }
}