Hi,
Just enable the "Order administrator notification" and the "payment notification" options under the "display: mail" section of the settings of your custom field and you should be good to go:
monosnap.com/file/Ra2zmJvGGbzxsujTSvXDQkZfWVZdNz
Note that if you have an old version of HikaShop, you might not have these display settings. In that case, you should update your HikaShop. I see that you have an old Essential edition, so you don't have access to the emails edition interface in the menu System>Emails which would allow you to edit the emails to add custom code and be able to display the custom fields even without that setting from recent versions of HikaShop.