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sandbox api - getting the correct url
This question often returns and I found the standard answer, but my case is slightly different.
So I know that the url to access the partner api (wsdl) is slightly different for a sandbox version (test.salesforce.com), however I can't change my initial url to connect, because I use the merge field {!$Api.Partner_Server_URL_160} to connect to the api using an already open session in salesforce. On the public urls for the partner api (either sandbox or production) you can only open a new session on the public wsdl and then receive a new url. As I don't have the log in credentials of the user I avoided this by using this merge field to get the correct url for an already connected user.
This doesn't work for a sandbox version. Any help or hint would be greatly appreciated.