For those wanting looking for an expression builder reference there isn't one, what you need is the language reference: https://developers.arcgis.com/arcade/function-reference/. This doesn't give the whole story away as you have to click through the links to discover that you can call geometry($feature).x and geometry($feature).y.
The following works for me, with an arithmetic alignment and a marker that points to 0 degrees (arithmetically):
var pX = geometry($feature).x;
var pY = geometry($feature).y;
var cX = 572730.054736;
var cY = 161931.381529;
var dX = cX-pX;
var dY = cY-pY;
Where cX and cY are the coordinates that I want the marker to point towards.