From https://pandoc.org/MANUAL.html#extension-yaml_metadata_block I see how to generate a pptx presentation from a markdown file
pandoc habits.txt -o habits.pptx
Title is set with
% title
% author(s) (separated by semicolons)
% date
How can I add a subtitle or another arbitrary text on the title page? I mean to add it to the title page of the slides, the .pptx file.
I think there is some confusion about the title block: what's specified in the question is a pandoc title block; it does not support subtitles.
YAML metadata blocks are the generic method of specifying document metadata. Including a subtitle is straight-forward this way:
---
title: title
subtitle: the elusive subtitle
author:
- Author 1
- Author 2
date: 2021-05-29
---
# Content slides start here