To open a serial port, first access a SerialPort object. For this, you can either prompt the user to select a single serial port by calling navigator.serial.requestPort() in response to a user gesture such as touch or mouse click, or pick one from navigator.serial.getPorts() which returns a list of serial ports the website has been granted access to.

document.querySelector('button').addEventListener('click', async () => {
  // Prompt user to select any serial port.
  const port = await navigator.serial.requestPort();
});

// Get all serial ports the user has previously granted the website access to.
const ports = await navigator.serial.getPorts();

If a serial port is provided by a USB device then that device may be connected or disconnected from the system. When the website has been granted permission to access a serial port, it should monitor the connect and disconnect events.

navigator.serial.addEventListener("connect", (event) => {
  const port = event.target;
  // TODO: Automatically open port or warn user a port is available.
});

Once you have a SerialPort object, calling port.open() with the desired baud rate will open the serial port.

// Wait for the serial port to open.
await port.open({ baudRate: 9600 });

Source: https://web.dev/serial/