ACF: Display custom metabox on an ACF (sub-) options page

acf-custom-metabox-on-options-page.php

<?php

/**
 * Add sub options page with a custom post id
 */
if( function_exists('acf_add_options_page') ) {
	acf_add_options_sub_page(array(
		'page_title' 	=> 'CSV Sync',
		'menu_title' 	=> 'CSV Sync',
		'parent_slug' 	=> 'users.php',
		'post_id'       => 'aa_ucs',
		'autoload'      => false,
	));
}

/**
 * Registers a metabox with ACF for a particular screen. You may need to find the screen ID yourself.
 */
function aa_ucs_register_acf_metabox() {
	// Verify the screen ID
	if ( !acf_is_screen( 'users_page_acf-options-csv-sync' ) ) return;
	
	// Add meta box
	add_meta_box( 'meta-box-id', __( 'My Meta Box', 'textdomain' ), 'aa_ucs_display_acf_metabox', 'acf_options_page', 'normal' );
}
add_action( 'acf/input/admin_head', 'aa_ucs_register_acf_metabox', 10 );

/**
 * Display custom metabox on an ACF options page
 */
function aa_ucs_display_acf_metabox() {
	$text = get_option( 'example-text' );
	?>
	<input type="text" name="exampletext" placeholder="Enter some example text to be saved" value="<?php echo esc_attr($text); ?>">
	<?php
}

/**
 * Submit metabox form, save the results
 * 
 * @param $post_id
 */
function aa_ucs_save_acf_metabox_fields( $post_id ) {
	if ( $post_id != 'aa_ucs' ) return;
	
	$text = isset($_POST['exampletext']) ? stripslashes($_POST['exampletext']) : false;
	
	update_option( 'example-text', $text );
}
add_action( 'acf/save_post', 'aa_ucs_save_acf_metabox_fields', 20 );