There are several options:
Send the original application for dismissal by international mail to the employer's official address. But in this case, you should take care of the evidence that you put the application for dismissal in the envelope (that is, have a description of how the envelope was enclosed).
Send an application for dismissal to the official e-mail address of the employer (direct supervisor and head of the enterprise). To do this, you should print and sign the application, scan it and sign the EDS and, together with the EDS protocol, send it to all known e-mail addresses of the employer. And be sure to request answers regarding the recipients' receipt of your application for dismissal in accordance with Art. 11 of the Law of Ukraine "On Electronic Documents and Electronic Document Management";
If it is absolutely not possible to send by mail or email, send a scanned resignation letter to all known messengers of the employer, including your supervisor.
But the employer can refuse to accept such a statement, since he will not be able to make sure whether you really sent it and whether it is really your will.