According to the documentation Preview URL (https://docs.squidex.io/guides/09-preview) supports placeholder.
However, it looks like that only one placeholder is supported.
Example:
Preview URL definition: https://matmatch.com/learn/${data.directory.en}/${data.url.en}
Actual results (on clicking the preview url on in the content item):
https://matmatch.com/learn/process/$%7Bdata.url.en%7D
so, the first placeholder war replaced, however, the second was escaped (not processed).
The Feature request is: increase the number of supported placeholders.