I would start by either using a parameter to pass in the users selection or retrieve it from the database by using a retrieve activity. Once you have the selected option you would be able to pass this to the location as Dirk as previously mentioned.
I would suggest that you set up a lookup for this that would allow you to configure the environment and associated location. This will make your solution more flexible. Your microflow could look something like this:
I would suggest creating the endpoint in a string variable in the microflow leading up to the webservice call.
Then you can simply input that variable in the service call.