You would have to use an AHK script to send the !{Enter} key combo after launch if it doesn't support any CLI params or use a settings or config file.
That being said, I've never heard of this emulator, have no idea what the "Online" refers to, and from a quick google search the only thing I found related to this emulator is some random blog post mentioning a private Facebook group, a random GDrive link to the supposed binary, and no source code.
Unless there's some compelling reason to use this over existing emulators like MAME or ParaJVE, I wouldn't bother. If there is, at minimum contact the dev and ask him to add a CLI param for fullscreen since that's trivial.