Matt, no it is not possible to test this within the player. You will need to have a built app installed on your device. You can install a built app and test the app's link by accessing the link from a browser. Doing this prompts you to launch the app.
I can, for example, write the app below, and give it an app link of applink1. When I build and install the app on my device, I can call it from a browser by visiting the link applink1://?latitude=40
import QtQuick 2.7
import ArcGIS.AppFramework 1.0
App {
id: app
width: 400
height: 640
Text {
id: txt
text: "No parameter called latitude exists"
anchors.fill: parent
wrapMode: Text.WordWrap
font.pointSize: 24
horizontalAlignment: Text.AlignHCenter
verticalAlignment: Text.AlignVCenter
}
onOpenUrl: {
var urlInfo = AppFramework.urlInfo(url);
var openParameters = urlInfo.queryParameters;
if (openParameters.hasOwnProperty("latitude")) {
txt.text = "Latitude is " + openParameters.latitude;
}
}
}