Use the old drupal_set_title('') function to set the title and put it in hook_form_alter. Why in a hook_form_alter? If you do it in preprocess page your 're to late, if you want it to do with drupal_set_title(). But what you can do in the preprocess page is set $vars['title'] (thanks to Davy [3] aka the drupalcoder [4] to point this out). Using hook form_alter has the advantage that it's independent of the theme.
Example:
function your_module_form_alter(&$form, $form_state, $form_id) { if ($form_id == 'aanvraag_node_form') { drupal_set_title(t('Schrijf je in als actievoerder')); } }
Links
[1] https://dominiquedecooman.com/blog-topics/drupal6
[2] https://dominiquedecooman.com/blog-topics/theming
[3] http://davyvandenbremt.be
[4] http://drupalcoder.com