Experiment: In the browser console, manually set the User ID

Here is something you can try in the console of any browser if you are visiting a page which uses the Matomo Tracker JS.

1. Open:
   Browser >> Dev Tools >> Console
2. Enter these 4 commands one at a time in this order, hitting <ENTER> after each one
3. _paq.push(["setUserId", anExampleVariable]); <ENTER>
4. let anExampleVariable = "The Man in the Moon"; <ENTER>
5. _paq.push(["setUserId", anExampleVariable]); <ENTER>
6. _paq.push([ function() {console.log(this.getUserId());}]); <ENTER>

Step 6 allows you to test whether Matomo stored the name of the User ID.

Then let's do this using your website where you have stored the name of your website visitor. We are going to take the visitor's name and store it as their Matomo UserID.

Let's assume your website allows the user to sign in, with a password, and the username is stored in a JS variable in the browser called first_and_last.

In a browser which has your user's web page load your website. Sign in as a username in your website:

1. Browser >> Dev Tools >> Console
2. Enter these 3 commands one at a time in this order, hitting <ENTER> after each one
3. first_and_last; <ENTER> just to see if your name is already in the page, nothing to do with Matomo yet
4. _paq.push(["setUserId", first_and_last]); <ENTER>
5. _paq.push([ function() {console.log(this.getUserId());}]); <ENTER>

Setting the UserID in Matomo has benefits:

• You can see the UserID immediately in Dashboard >> Visits >> Visits Log
  
• You can see a special report on signed in users at Dashboard >> Visitors >> User IDs
• You can detect each history of each visitor on your site, including the monetary value of their conversions.  (You have to set up the conversions manually - see links below. Once you set these up, you can start measuring the rich data of how often and how much are the conversions worth, over time, for this visitor.)
  

Matomo uses three ways to string together repeated visits over time for a visitor. The weakest but most legal (most private) is the scrambled fingerprint number. This is remembered only for today (actually less than today: for privacy, this ID is deleted within 30 minutes of the session end.) You can make this time longer than 30 minutes but it puts a high demand on your CPU if you try to set it for several days duration. Still, this might work for a low traffic website.   It will not work for Matomo for Wordpress: those deployments have small resources in the hardware: Matomo has to share CPU and SQL database with the WP resources. Not good.

Stronger than this is detection by first-party cookies.  By default, Matomo will do this. If you want to be super private with your visitors data you should disable this.

Strongest of all is detection by User ID. UserID is what we experimented with at the top of this article.

Read more at the official Matomo Docs:
How does Piwik detect unique and returning visitors? (with User ID or Visitor ID from tracking cookie)
https://matomo.org/guide/reports/user-ids/
https://matomo.org/guide/reports/goals-and-conversions/