JSFiddle - React, Tailwind, and code Playground

HTML

<form name="Asking">
                       <label id="TextBoxForKeyword">
                              キーワード:<input type="text" name="KeyWordEdit" list="KeywordsList">
                       </label>
                       <datalist id="KeywordsList">
                                 <option value="選択肢A"></option>
                                 <option value="選択肢B"></option>
                                 <option value="選択肢C"></option>
                                 <option value="選択肢D"></option>
                                 <option value="選択肢E"></option>
                       </datalist>
               </form>

JavaScript

document.Asking.addEventListener('submit', e => console.log( Array.prototype.map.call(e.target[0].list.options, elm => elm.value).indexOf(e.target[0].value)))