browser developer tools on the network tab have copy as curl value, however it seems still no luck:

curl 'ws://localhost:8080/' -H 'User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:138.0) Gecko/20100101 Firefox/138.0' -H 'Accept: */*' -H 'Accept-Language: en-US,en;q=0.7,id;q=0.3' -H 'Accept-Encoding: gzip, deflate, br, zstd' -H 'Sec-WebSocket-Version: 13' -H 'Origin: null' -H 'Sec-WebSocket-Extensions: permessage-deflate' -H 'Sec-WebSocket-Key: 3tMWYzcHwmObiLR3nWOgsg==' -H 'Connection: keep-alive, Upgrade' -H 'Sec-Fetch-Dest: empty' -H 'Sec-Fetch-Mode: websocket' -H 'Sec-Fetch-Site: cross-site' -H 'Pragma: no-cache' -H 'Cache-Control: no-cache' -H 'Upgrade: websocket'
curl: (22) Refused WebSockets upgrade: 400

okay, changing Sec-WebSocket-key with the one provided from browser works (not sure why):

curl --include \
     --no-buffer \
     --header "Connection: Upgrade" \
     --header "Upgrade: websocket" \
     --header "Host: localhost:8080" \
     --header "Origin: http://localhost:8080" \
     --header "Sec-WebSocket-Key: 3tMWYzcHwmObiLR3nWOgsg==" \
     --header "Sec-WebSocket-Version: 13" \
     http://localhost:8080/
HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: YLxhg+bEzrZXCaOyyI5QkRBIdT4=

�       something

EDIT:
after reading the wikipedia, turned out it just a random 16-byte encoded with base64, on Linux, you can generate it with:

head -c 16 /dev/urandom | base64

This was very helpful. Thanks so much. In my case I just needed to confirm that the websocket server could be connected to and the connection would be upgraded, but I see how the headers and other settings can be changed as needed for any particular situation.