You can pass the path to wiki, which will open at the program start, through the command line. For example:
outwiker.exe "C:\temp\samplewiki"
or
outwiker.exe "C:\temp\samplewiki\__page.opt"
If the tree notes should be open as "read-only", you need to add the option --readonly or -r:
outwiker.exe "C:\temp\samplewiki" --readonly
or
outwiker.exe "C:\temp\samplewiki" -r