In the “URL commands” section, it would be helpful if parameters where marked with “required”, when they are. E.g., the text parameter for createMarkdown: it is apparently not possible to create an empty MD file with x-devonthink://createMarkdown?title=Title, text is required.
Actually, creating an empty Markdown file with exactly the URL you provided, is possible. It works exactly as expected.
For more flexibility and less app switching you may want to have a look at the new Shortcuts support.