Is it possible to define lifecycle methods in Vue 3 while using the <script setup> syntax?

I have defined a SFC similar to the one below

<script setup>
const msg = 'Hello World!'
const props = defineProps({....})
.....
function onMounted() {
    console.log('the component is now mounted')
}
</script>

<template>
    <p>{{ msg }}</p>
</template>

The onMounted function is not executing at all.

I wasn't able to find anything in the Vue documentation. Is it even possible to declare lifecycle hooks like this?