The correct way to do this is to use the Navigate > Open an external website Action, and construct the URL yourself as a text expression. Note that there is a handy built-in expression (“Website home URL” which resolves to the correct base URL for your site, including things like /version-test/ and /version-live/, so you don’t have to work out the logic for that. Note that “Website home URL” INCLUDES the trailing slash, so don’t add one manually. The slug you construct should not start with “/”, but with page name).
So, to go to some page in your site:
If you need to send a query string or a Thing’s unique ID, you construct that yourself also.