JSFiddle - React, Tailwind, and code Playground

HTML

<form action="sendEmailScript.php">
    <input type="text" name="sampleFiled" />
    <input type="hidden" name="jsArray" />
</form>

JavaScript

var myArrayToSend = ["val1", "val2"]
var arrayField = document.querySelector("[name=jsArray]");
arrayField.value = JSON.stringify(myArrayToSend)