When uploading a template within the Salesforce Lightning component, click on the View Merge Fields button.

This will open a list of merge fields that you can use to populate your template.

When you have clicked on the button you will see a list of the merge field you can copy and see if they also have a value to them.

You can copy them by simply clicking on the orange button to the left by each field.

Custom fields will appear here as well. Depending on the record type you will see different merge fields available.

When you are in your GetAccept template, simply click anywhere within your editor block where you want to add your merge tag.

Then click on the { } symbol in the menu