Hey man I've a request in Single Play when you select a custom Gameport (yellow star) we are unable to writte a custom starting map for example test01 and so we are forced to start on officials map like map01 instead, but that option is available on official ports like GZDoom or Zandronum so could you please fix that problem?

All settings are transferred to the program via the command line. For example map name for Zdoom-based ports is set so: "+map mapname".
For custom port I dont know how to do that - it depends on port command line interface, so only standard 'vanila doom' parameters are used (and not necessary port support them).
If you know these parameters for your port - add them to 'Additional command-line parameters' list