The thread below has information.  Though the manual makes no mention of USB-C charging, apparently the system does support it. If your system has integrated video (Intel), 65W is sufficient and should work. If your system has discrete video, it needs the 90W AC adapter -- and it's unlikely to run at 90W from any USB-C charging adapter (it may still charge, but the system won't run at full speed).
Hello good people, I want to buy a USB-C charger for my Dell Latitude 7480 laptop. What should I look for when buying a USB-C charger?   I will really appreciate your help. Thank you in advance 
Dell Latitude 7480 - Help with buying USB-C charger