There is a nice article that shows what you need to do on the Pega 7 side first.  Check out the section on configuring the VM here: https://collaborate.pega.com/discussion/rest-service-integration-guide-tutorial

With this step done, now move to the Pega Robotics side.  You need to do several things.  First, make sure that the menu item for RPACredentials is enabled in the RuntimeConfig.xml.  It should look like this:

While editing the RuntimeConfig.xml you also need to set the path to the PegaServer in the Robotics section.

Next, you must provide the Pega Robotics Runtime with the Administrative Operator you created in Pega 7.  This done by running the Pega Robotics Runtime and right-clicking on the tray icon.  Select Enter RPA Credentials.  This only needs to be done once or if you need to change the Administrative Operator.

Your solution determines which WorkGroup the robot will register with.  On the properties page of the solution set the Robot Work Queue to the WorkGroup the solution should service.  This is case-sensitive so be sure you type the WorkGroup exactly as it is defined in Pega 7.

The final step to register your robot is to run the solution using the Pega Robotics Runtime.  Please note that registration will not happen if you run the solution using Pega Robotics Studio.  Check in Pega 7 for an Operator ID addded with the name of your VM to be sure that the registration was successful.