Custom e-mail field

Hey there.

I would like to add a custom field to the mail a customer receives when his order is completed.

What I want to achieve is an easy way for me to input a tracking number in a field when sending the completed mail, which should then insert the given tracking number at a specific place in the mail and send it to the customer.

How would this be possible?

Thanks already,

https://www.ads-software.com/plugins/woocommerce/