MyInput.vue

<template>
    <div>
        <p> Wrapper for a text input </p>
        <input 
            type="text"
            placeholder="Custom input!"
            @change='customChange'
        />
    </div>
</template>

<script>
export default {
    setup (props, context) {
        const customChange = (event) => {
            context.emit("customChange", event.target.value)
        }

        return {
            customChange
        }
    }
}
</script>

<!-- OR -->

<script>
export default {
    setup (props, { emit }) {
        const customChange = (event) => {
            emit("customChange", event.target.value)
        }

        return {
            customChange
        }
    }
}
</script>