Yes the copper tube will be for a mechanical gauge, there must have been one in your car at one time. The copper tube is fastened into the housing with a 1/4 inch [1/8?] NPT nut. Turn out the copper tube and replace it with the electric sender switch. In the pic below I have a brass block screwed into the adapter then a tube for a mechanical gauge and an electric switch for the light. I could unscrew the block and screw the switch right into the adapter. The tube in your adapter will, or should, be screwed into the adapter with some kind if nut to hold it in place. Just turn out the tube's fastener and remove it to gain access to the port for the sending switch.