axiosでpostした時にError: Request failed with status code 400のエラーが出る

前提・実現したいこと

Firebaseを使って新規登録の処理を実装したい。

発生している問題・エラーメッセージ

Uncaught (in promise) Error: Request failed with status code 400

該当のソースコード

↓axios-for-auth.js

javascript
1import axios from 'axios';
2
3const instance = axios.create({
4    baseURL: 'https://identitytoolkit.googleapis.com/v1'
5});
6
7export default instance;

↓main.js

javascript
1import { createApp } from 'vue';
2import App from './App.vue';
3import router from './router';
4import axios from 'axios'
5import VueAxios from 'vue-axios'
6
7const app = createApp(App);
8app.use(router);
9app.use(VueAxios, axios)
10app.mount('#app');

vue
1<template>
2  <div>
3    <h1>新規登録画面</h1>
4    <form>
5      <ul>
6        <li class="user-name">
7          <label for="user-name">ユーザー名</label>
8          <input type="text" id="user-name" placeholder="UserName" v-model="username" />
9        </li>
10        <li class="email">
11          <label for="email">メールアドレス</label>
12          <input type="email" id="email" placeholder="E-mail" v-model="email" />
13        </li>
14        <li class="password">
15          <label for="password">パスワード</label>
16          <input type="password" id="password" placeholder="PassWord" v-model="password" />
17        </li>
18      </ul>
19    </form>
20
21    <button @click="register">新規登録</button>
22    <br />
23    <router-link to="/" tag="a">ログインはこちらから</router-link>
24  </div>
25</template>
26
27
28<script>
29import axios from '../axios-for-auth.js';
30
31export default {
32  data() {
33    return {
34      username: '',
35      email: '',
36      password: ''
37    };
38  },
39  methods: {
40    register() {
41      axios
42        .post('/accounts:signUp?key=myKey', {
43          email: this.email,
44          password: this.password,
45          returnSecureToken: true
46        })
47        .then(response => {
48          console.log(response);
49        });
50      this.username = '';
51      this.email = '';
52      this.password = '';
53    }
54  }
55};
56</script>