To specify which topic file should open first in a browser when you click View Output in the DITA2Go Project Manager:
[HTMLOptions] ; ViewOutputFile = file in wrap directory for Project Manager to open ; in browser upon View Output. ViewOutputFile = filename.ext
The file you specify must be present in the wrap directory; see §44.6.1 Specifying a wrap directory. Or, if you include a path, the path must be relative to the wrap directory.
By default, the Project Manager uses the value of [Contents]TOCFile, and looks for a file of that name in the wrap directory (see §14.3.2 Specifying a file name and title for the TOC). If TOCFile is not set, or the named file is not present in the wrap directory, the Project Manager looks in the wrap directory for a file that has the map name for your project and a FileSuffix extension (see §4.1.6 Checking output type and file extension).
The Project Manager opens the starting topic file in the default browser. To specify a different browser or viewer:
[HTMLOptions] ; ViewOutputCommand = path\to\browser, default none
See View-output command under §1.3.7 Establish system-wide configuration settings.