Adding a custom field in emails

Hi, so when purchasing from my website there is no shipping, however the customers come and pick it up when it’s ready. When they get the “Completed mail” there should be a field that says “Pick-up date” or something like that so the customers knows when to pick it up. I was thinking i could use a custom field in the individual orders and add the custom field to the email, but can’t get it working. If anybody could help it would be very much appreciated.

Thanks

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