It depends on the version of RoboHelp you are using. It can be done in RoboHelp 9 as explained in the sample projects.
Click Open on the RoboHelp Starter page and then click Samples in the ribbon on the left.
Please also see the Sticky Topic "Before you Post" and always start posts with the edition and version of RoboHelp being used.
Come back if still stuck.
See www.grainge.org for RoboHelp and Authoring tips