I want to requesting for POST but ıt2s be OPTION - api

Question

Hello i want to POST request with Axios to my back-end's API on VueJS. But my POST request turn a OPTIONS request.What i do wrong to where? Can you help me? i will go crazy...
Signup.vue Code:
<script setup lang="ts">
import axios from 'axios'
import { ref } from 'vue'
const fullName = ref('')
const username = ref('')
const email = ref('')
const password = ref('')
async function addUser() {
const response = await axios.post("http://localhost:3000/user/", {
fullName: fullName.value,
username: username.value,
email: email.value,
password: password.value
})
console.log(response)
}
</script>
<template>
<div class="">
<h1>Login</h1>
<br>
<label for="fullname">Name & Surname</label>
<input type="text" v-model="fullName" id="fullname" class="form-control " required autofocus placeholder="John Die" aria-label="Fullname" aria-describedby="basic-addon1">
<label for="username">Username</label>
<input type="text" v-model="username" id="username" class="form-control " required placeholder="john.die" aria-label="Username" aria-describedby="basic-addon1">
<label for="email">E-mail (optional)</label>
<span class="input-group-text" id="basic-addon1">#</span>
<input type="text" v-model="email" id="email" class="form-control" placeholder="johndie#example.com" aria-label="E-mail" aria-describedby="basic-addon1">
<label for="password">Password</label>
<input type="password" v-model="password" class="form-control" required placeholder="Password" id="password">
<button name="button" #click="addUser()" class="btn btn-success text-center input-group" >JOIN</button>
<br><br>
<p class="text-center">
Do you have an account?
<a id="login" href="/login" class="text-info">Login</a>
</p>
</div>
</template>