How to add my 3rd party Checkout fields?

Hi,

I just purchased “WooCommerce Email Template Customizer” and it doesn’t work as I expected. In your customizer, there are no short codes from my “WooMatrix” Checkout fields. But It’s important for me to add this customizes fields to the E-Mails. If can’t, your Plugin is useless for me.

Please tell me, how I can add the extra Checkout fields.

Regards

Gerd