Advanced Custom Fields hidden field does not have a default value input.
Add one by implementing the code below. Personally I was using this to mark a registration form for a particular user type.
<?php
// add default value to hidden field
add_action('acf/render_field_settings/type=hidden', 'tb_hidden_default_render_field_settings');
add_filter('acf/prepare_field/type=hidden', 'tb_hidden_default_prepare_field');
function tb_hidden_default_render_field_settings($field){
acf_render_field_setting( $field, array(
'label' => __('Default Value'),
'instructions' => '',
'name' => 'hidden_default',
'type' => 'text',
'ui' => 1,
), false);
}
function tb_hidden_default_prepare_field( $field ) {
if( empty($field['hidden_default']) ) return $field;
$field['value'] = $field["hidden_default"];
$field['label'] = '';
return $field;
}