JSFiddle - React, Tailwind, and code Playground

HTML

<select id="selectionType">
    <option value="-1">Select One</option>
    <option value="0">First Name</option>
    <option value="1">Last Name</option>
</select>
<input type="text" id="inputBox">

JavaScript

var placeholderText = {
    "First Name": "Enter your First Name",
    "Last Name": "Enter your Last Name"
};

$("#selectionType").on("change", function () {

    if (this.value != -1) {
        $("#inputBox").val(placeholderText[$("#selectionType option:selected").text()]);
    } else {
        $("#inputBox").val("");
    }

});