Answer
That colored background is a separate "emphasis" setting applied to the markdown control, not something achievable through the markdown content itself — and emphasis for markdown controls isn't currently user-configurable, so there's no way to reproduce it on your own markdown text through configuration. For an easier authoring experience than the small plain-text input field Page Designer provides, write and format the markdown in an external markdown editor first, then paste the result into the field — just remember to manually convert line breaks to \r\n before pasting, since Page Designer's field won't interpret plain line breaks as markdown line breaks on its own.