https://www.tenable.com/products/nessus/activation-code
In the "Nessus Essentials" section, click "Register Now", as shown below.
Fill out the form with your name and email address and click the Register button.
On the next page, click the Download button.
Click the appropriate download link, as shown below.
Agree to the agreement.
A Web page opens, as shown below.
Click the "Connect via SSL" button.
On the next page, approve the SSL exception. If you are using Internet Explorer, that's done by clicking "Continue to this website (not recommended)".
When it asks you which version to use, accept the default selection of "Nessus Essentials", as shown below.
At the "Get an activation code" screen, click Skip. Read your email to find an activation code and enter it into the Nessus installer.
Proceed through the installer. When you're done, it takes a long time (15-30 min.) to download and install components, as shown below.
When it's ready, you see the "My Scans" page, as shown below.
IPCONFIG
In the "Scan Templates" page, click "Basic Network Scan", as shown below.
Enter a Name of "basic" and a Target of your IP address, as shown below.
At the lower left, click the Save button.
In the "My Scans" page, on the "basic" line, on the right side, click the right-arrow to launch your scan, as shown below.
The scan takes about five minutes. When the scan finishes, a gray check mark will appear in its line, as shown below.
Click "On Demand".
The results appear, as shown below.
They are color-coded.
Click the Vulnerabilities tab to see details, as shown below.
In the results, click the Vulnerabilities tab. Click "Nessus SYN scanner", as shown below.
Four ports are open. Find the port number of the highest open port. That's the flag.
Posted 9-6-18
Added installation of dirmngr 10-10-18
Removed reference to Debian 11-6-18
Ported to new scoring engine 7-12-19