index.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <div>
        <input type="text" id="name">
        <input type="submit" id="handelName" value="Submit">
    </div>

    <script>
        const btnUser = document.getElementById("handelName")
        btnUser.addEventListener("click", ()=> {
            const name = document.getElementById("name").value
            fetch('http://localhost:4200/addItems', {
                method: 'POST',
                body: JSON.stringify({
                 name: name
                }),
                headers: {
                  "Content-type": "application/json; charset=UTF-8"
                }
              })
              .then(response => response.json())
              .then(json => console.log(json))
        })

    </script>
</body>
</html>