Custom Subject

Hello support,

Need to know about I want to send email on custom order status and its encoded in processing status. So when processing email goes I want to send custom subject with it from functions.php

add_action( ‘woocommerce_email_before_order_table’, ‘mm_email_after_order_table’, 10, 4 );

currently I am using above code for to attach some extra fields with it. I want the custom subject line also please provide guideline.