Hey
I just copied from another post in the forum and as you can see they console.log(uploadedFile.url) and that is the url the file / document got so use that.
$w("#uploadButton1").startUpload()
.then( (uploadedFile) => {
console.log("Upload successful. File is available here:");
console.log(uploadedFile.url);
$w("#dataset1").setFieldValue("image1", uploadedFile.url);
$w("#dataset1").save()
.then( (item) => {
console.log("Save successful");
let fieldValue = item.item1;
} )