in the rare time they host a server, which you may hear about from subscribing/following certain things. for myself I learned about this in an achievement focused steam group and it popped up in my friend activity feed on steam community. this discord server appears to be another avenue:
https://discord.com/invite/halflifeso firstly, lower your volume to a cautionary level. most important step
next, have half life 2 and the synergy mod installed. the event today asked us to enable some developer branch? you can find that in properties from right-clicking the mod in your steam library
the server may have a password and to make it easy they might offer a console line you can copy and paste to reconnect. there may be server troubles, including disconnections or supposed reserved player slots. so what I'm leading to is you will need to enable the console command key, which you will find in keyboard bind options
so if we connect successfully you must enjoy or suffer for about five minutes of gameplay. I have witnessed folks complaining about trying to 1 to 3 hours, perhaps I was lucky to step in when things ironed out. good luck and have fun(?)
[I am very open to feedback on my joke of a guide]