This was a great question and solution!
I used it to pull the coordinates for 2 arcade expressions representing the start and end points of the line. I didn't try it in fieldmaps but I think it would work to have it calculate column values for the line start/end points but I don't think you can have it create a point that represents the start/end point.
//Find the start coordinates
var xval = Geometry($feature).paths[0][0].x
var yval = Geometry($feature).paths[0][0].y
function metersToLatLon(xval, yval) {
var originShift = 2.0 * PI * 6378137.0 / 2.0;
var lon = (xval / originShift) * 180.0;
var lat = (yval / originShift) * 180.0;
lat = 180.0 / PI * (2.0 * Atan(Exp(lat * PI / 180.0)) - PI / 2.0);
return [lat, lon];
}
var latlon = "";
var result = "";
var latlon = metersToLatLon(xval, yval);
result = Round(latlon[0], 6) + ', ' + Round(latlon[1], 6);
return result;
//Find the end coordinates
var xval = Geometry($feature).paths[-1][-1].x
var yval = Geometry($feature).paths[-1][-1].y
function metersToLatLon(xval, yval) {
var originShift = 2.0 * PI * 6378137.0 / 2.0;
var lon = (xval / originShift) * 180.0;
var lat = (yval / originShift) * 180.0;
lat = 180.0 / PI * (2.0 * Atan(Exp(lat * PI / 180.0)) - PI / 2.0);
return [lat, lon];
}
var latlon = "";
var result = "";
var latlon = metersToLatLon(xval, yval);
result = Round(latlon[0], 6) + ', ' + Round(latlon[1], 6);
return result;