First the_field() is only used to display fields. To use the value of the field somewhere else you need to use get_field()
You also need the post ID of the post where that other field appears. Basically, what you need is a post object field that returns the ID and the field name you want to put from that page.
Then, in long form
// get the field and post ID from this post
$field_name = get_field('field name of field containing field name');
$post_id = get_field('field name of post object field that has the above field');
// show the value from that other post
the_field($field_name, $post_id);
