Question: Is it possible to redirect from an existing application to an appsmith app with parameters in the URL being read before the app screen loads?
Yes, it is possible to pass query params to the URL and catch it in JSObject or in moustache bindings on page load using appsmith.URL.queryParams.param
Note that running JSObjects on page load is only available in the cloud version at the moment and will be released soon to self-hosted users as well.