Create a billboard

Create a new billboard.

    ### Parameter Options & Tips:
    - **body_markdown**: The HTML/Markdown advertisement copy.
    - **placement_area**: Target region in layouts (e.g. `post_comments` below comments, `sidebar` in sidebars, `home_feed` between posts).
    - **display_to**: Cohort target rules (e.g. `all` for everyone, `logged_in`, `guests`, or customized segments).
    - **target_geolocations**: Comma-separated ISO codes for country/region targeting.
    - **approved** & **published**: Set to `true` to activate billboard rotation instantly.