In my case, I went to Add your own columns and added a column called Subtitle with the lookup name of #subtitle. The column type is text and column is shown in the Tag browser. When editing metadata, this column shows in the Custom metadata tab.
On the Metadata, on device & advanced tab of the Kobo Touch or Kobo Touch extended driver configuration, I checked Subtitle and added {#subtitle} as the template.
Anything you put into the subtitle field will then show up on your Kobo as the subtitle.