So, after looking through the api a little more, I found the "displayField" property. That did the trick.
var trailheadsLayer = new FeatureLayer({
url: "https://services6.arcgis.com/708LZoWlLL5mAZXZ/arcgis/rest/services/trailheads/FeatureServer",
popupTemplate: trailHeadPop,
displayField: "TRL_NAME",
outFields: ["*"]
});
Thanks for your help and apologies for not reading the api a little more thoroughly before posting!