To start a NoMachine connection, the NoMachine client reads the connection settings stored in the NXS files, which by default are in the user's Documents/NoMachine directory.

From the command line all you need to do is run the player and pass the session file with: 

<path to nxplayer> --session <session file >

By default, on Linux:

/usr/NX/bin/nxplayer --session <path to .nxs file>

on macOS:

/Applications/NoMachine.app/Contents/MacOS/nxplayer --session <path to .nxs file>

on Windows:

it's possible to create a link to the nxplayer program and set the option to execute it in the property tab of such link.
Otherwise, to execute the client from a CMD console:

C:\Program files (x86)\NoMachine\bin\nxplayer --session <path to .nxs file>

If the connection file is pre-configured for running a custom session in floating-window mode, it's possible to specify the --hide option to hide the client window when connecting. The client window will appear again when the session disconnects. For example:

/usr/NX/bin/nxplayer --session <path to the NXS file> --hide

For more information on how to pre-configure the session file and other client settings, please consult:

https://kb.nomachine.com/DT07S00240 for v. 8
https://www.nomachine.com/DT11R00186 for v. 7
https://www.nomachine.com/DT10O00159 for v. 6
https://www.nomachine.com/DT07M00094 for v. 5