If your domain is currently pointing to your Droplet IP and is accessible, you should be able to use the domain and prefix the port to access Plesk.
Plesk uses SSL by default, so you may see a security warning unless you've configured an SSL certificate for your domain.
For example, if my domain is
domain.com, I can then use the following URL to access Plesk:
If you've not yet added your domain to Plesk, you can click on Websites & Domains from the left side panel, followed by the Add Domain button. Once the domain is added, update the A entry for your domain to point to the IP of the Droplet where Plesk is installed and allow the DNS to resolve.