The Wrap=placeholder feature was not working on the Horizon theme. pfaffman reported the issue and provided examples of the code not working. However, Arkshine found it to be working for them. After switching to the default theme, pfaffman confirmed that the feature was working, suggesting the issue was theme-specific. Lilly later added the placeholder component to the Horizon theme, resolving the issue.
[wrap=placeholder key="two" description="Subdomain of first multisite site" default="two"][/wrap]
### Suddomain for the 3rd site
[wrap=placeholder key="three" description="Subdomain of second multisite site" default="three"][/wrap]
It should be working now - I just added the placeholder component to the Horizon theme. Thanks for the heads up, that was me who forgot to include it when I installed the Horizon theme on Meta.