1. Make sure there's -usercon in startup. In default server startup line looks like

./srcds_run -game csgo -console -port {{SERVER_PORT}} +ip 0.0.0.0 +map {{SRCDS_MAP}} -strictportbind -norestart +sv_setsteamaccount {{STEAM_ACC}}

change to

./srcds_run -game csgo -console -usercon  -port {{SERVER_PORT}} +ip 0.0.0.0 +map {{SRCDS_MAP}} -strictportbind -norestart +sv_setsteamaccount {{STEAM_ACC}}

2. create /csgo/cfg/server.cfg file with following content

rcon_password "15aa7022f65be999d404432c76b3fa67"
hostname "My new server"

3. Restart server

To connect inside csgo game console type (replace IP:PORT from control panel)

rcon_address IP:PORT

rcon_password 15aa7022f65be999d404432c76b3fa67

verify connection with

rcon status

It should output:

] rcon status
hostname: My new server
version : 1.38.4.6/13846 1529/8749 secure  [G:1:3827339]
udp/ip  : 0.0.0.0:9274  (public ip: xx.xx.xx.xx)
os      :  Linux
type    :  community dedicated
map     : de_dust2
players : 0 humans, 0 bots (20/0 max) (hibernating)