Need to call 2 methods to retrieve data

Hi,

I'm new to Jaspersoft, so my apologies in advance if my question applies more broadly and I'm posting in the wrong place. I'm trying to use a web service in Jaspersoft that needs a call to its Login function first. That function receives a username and password, and returns a temporary access token. That token can be used to issue another function call later that will return actual data for the report.

So far I've done the following:

1. I created a "Login" sub-report. Use the web service data adapter, point it to the Login method, provide credentials etc. The token gets stored in the field called "token".
2. I added a variable to that sub-report, called "accessToken", whose Expression is set to $F{token}. I can see that the correct value gets stored if I add a text field and set its expression to $V{accessToken}.
3. I created the main report and added the sub-report to the first band. I also added an "accessToken" variable there as well. For sub-report properties, under Return Values, I set From and To to "accessToken", hoping that main report's access token varialble will be populated with the data from the sub-report.
4. Lastly, in the Query Dialog of the main report, I set the Base URL to something like GetEmployees?accessToken=$V{accessToken} - this should return a JSON of a list of employees, if the access token is valid. However, when I try to get the server's response, I get an exception, and it looks like $V{accessToken} doesn't get substituted with the actual token value.

Is my approach generally correct? How can I use the data obtained from a sub-report in a call to another web service function? Thank you!