Lack of information here thwarts anyone's ability to help...What AMD card specifically? What power supply is in that PC? What other parts (CPU) are in there? No idea what a dell optiplex 780 can have in it, bud. After taking the time to look it up, it seems quite old with old hardware....... if you have a new UEFI GPU it may not work.
There isn't a setting to get it to work, no. Just plug it in.
To be clear, you've installed it, connected any power plugs going to the mystery GPU and now do not have a picture on screen, right? Does the system seem like it is powering on?