Before you start with the Power Automate flow and the Power Virtual Agent setup you need to make sure you request a 30-day trial API key at GetAddressIO. Obviously there are also other services out there. For me this was just the easiest one to setup for a demo.
Also be aware that this is using a premium connector in the Power Automate flow, which might require an additional license.
The flow steps
1. Create a flow with a When Power Virtual Agents calls a flow trigger action. Add a text Postcode input field and a House number input field to that action.
2. Add three Initialize variable actions. Below is a table with the name, type and value for each of the variables.
Your key from the trial
3. Add a HTTP action. Use the GET method to interact with the Find Method of the Get Address IO API. Use the URI from below.
4. Add a Return value(s) to Power Virtual Agents action. Use the following expression. In this expression I am retrieving the 10th item of the result set. It probably makes more sense to retrieve the first item. In that case replace  by the index reference .