Install WiaTag app on the user's phone, add a unit in your account with the WiaTag device type and a random ID. Login into the Configurator for the WiaTag application using your Wialon credentials. You should see there your newly created unit, click it and select the working mode (e.g. Active for more data and smoother tracks or Light for battery saving). Activate option «Generate QR code». Save the changes. A pop-up window should appear with the QR code that you can scan with the WiaTag application, or copy a link with the settings and send it to the user via any messenger. By clicking on that link, the user should automatically get the necessary configuration.
Thus, you should get a new unit that is displaying user's location.
Configurator — https://apps.wialon.com/#configurator
Next, create a notification for the vehicle (not WiaTag), select the notification type «Interposition of units». Select option «Approaching», choose the WiaTag unit from the list and redefine radius (1000 m by default). In the next window, define the actions, e.g. send a Command. You can keep the rest of the settings unchanged.
Interposition of units — https://wialon-help.link/8164d509
Once the user with his phone is approaching a vehicle, the notification triggers and sends a command.
I recommend making a test with one or two units, to make sure this solution works.