Add a custom order note programmatically in Woocommerce admin order edit pages

From a dynamic Order Id you can use WC_Order add_order_note() method this way:

// If you don't have the WC_Order object (from a dynamic $order_id)
$order = wc_get_order(  $order_id );

// The text for the note
$note = __("This is my note's text…");

// Add the note
$order->add_order_note( $note );

Tested and works.

Thank you guys I was trying to find a way to add the notes to a new order. I was looking for the right hook using the solution that @LoicTheAztec posted. This is the solution that worked for me hope it helps someone else out there.

add this to the Functions.php file

add_action( 'woocommerce_new_order', 'add_engraving_notes',  1, 1  );

function add_engraving_notes( $order_id ) {
 //note this line is different 
 //because I already have the ID from the hook I am using.
 $order = new WC_Order( $order_id ); 

 // The text for the note
 $note = __("Custom Order Note Here");

 // Add the note
 $order->add_order_note( $note );

}