Set the initial value of a Select from fetched data

<Component name="Test" var.selectedValue="">

<DataSource
  id="myData"
  url="/api/users_initial_value"
  onLoaded="(data) => { selectedValue = data[0].id }"
/>

<Select initialValue="{selectedValue}">
  <Items data="{myData}">
    <Option value="{$item.id}" label="{$item.username}" />
  </Items>
</Select>

</Component>