You can use the filepicker.store() API for exactly this purpose - https://developers.inkfilepicker.com/docs/web/#store
var input = document.getElementById("store-input");
filepicker.store(input, function(InkBlob){
console.log("Store successful:", JSON.stringify(InkBlob));
}, function(FPError) {
console.log(FPError.toString());
}, function(progress) {
console.log("Loading: "+progress+"%");
}
);