Now, after adding these code you have to add a hidden field into your form. You have to insert the meta key name "wpuf_hidden_cat" and the value will be your category ID.

set default category for a form

add_action( 'wpuf_add_post_after_insert', function( $post_id ) {
wp_set_post_terms( $post_id, get_post_meta( $post_id, 'wpuf_hidden_cat', true ), 'your taxonomies name' );
});