Adding Predefined Variables to New Email Template?

I am creating a new email template and am trying to attach predefined variables (specifically Order Id) to the new template. I would like the option of clicking on the “Insert Variable” button and being able to insert the Order ID variable into the Template Content section.

However, I am not given the option to select that Order ID variable.

I am given the Order ID variable in the New Order template so when I started creating the new template I loaded the New Order template in but it still didn’t show me the Order ID variable.

I’ve looked through a few similar questions but they were a bit older and did not apply.

Thanks.