So if you didn't already know how to open the developer console, first press the escape key and then go to options while you're already in a server and then press advanced FROM THE KEYBOARD TAB now once you've done that, click enable developer console and now you can press ` (which most likely should be below the escape key on your keyboard) to open the dev console.

Now yes, if you're on a public server and the cheats are disabled im sorry man you can't spawn it unless you somehow make the server owner enable cheats in the server (if you are the owner of a server you can enable cheats in it if you want obviously)
This is the command to type into the dev console to enable cheats: sv_cheats 1
Now once you've done that type: prop_physics_create props_c17/oildrum001_explosive.mdl
and bam it'll spawn the flammable oil barrel