Redmine

Hello, I'm new to Redmine and could not find answers for some customized solutions. In my job, we're trying to use Redmine for controlling a business process, so we need to do some integrations.
Is it possible to populate a field based on a webservice answer?
For example, I have two combobox STATE and CITY, how can I call a webservice to give me the list of cities based on user option for a STATE and fill the CITY combo?

Thank you,

Marcio