This is where the specific "web-dl" technique comes into play. You need to utilize the dart:html library (or dart:js_interop for newer Dart versions) to initiate the download.
web-dl.fly3rs appears to be an unregistered or non-standard domain (as of this check). If encountered in network logs or emails, it may indicate a potential phishing or malware distribution site mimicking video download services. web-dl.fly3rs