We are talking CHM files here right? If so, go into your Window properties and click on the Advanced Properties button. In there is a Home field that can be used to enter a URL for a webpage. This can be a topic file name inside the same CHM (e.g.topica.htm). Note the home topic can be different from the default topic.
Thanks for pointing me in the right direction. I had something in the field there about home page but I may have got the path slightly wrong. After a bit of fiddling it now works. Thanks.
It would be nice there to be able to browse to a topic rather than having to manual type in a path.
You may want to submit a feature request for this. You can do so here. The problem as far as the UI is concerned is that the field can also be used to open a web page in which case a Browse button would be of no use.