const titleInput = useTemplateRef('titleInput');

onMounted(() => {
  nextTick().then(() => {
    titleInput.value?.inputRef?.focus();
  });
});

<template>
...
<UInput
  ref="titleInput"
  v-model="challenge.title"
/>
...
</template

const titleInput = useTemplateRef('titleInput');

onMounted(() => {
  nextTick().then(() => {
    titleInput.value?.inputRef?.focus();
  });
});

<template>
...
<UInput
  ref="titleInput"
  v-model="challenge.title"
/>
...
</template