Form custom development in v4

Regarding the new v4 theme templating language, I no longer see an easy or obvious way to manipulate the custom form fields on the new request page of guide.

Previously I could target a field by its cusotm field id.

Example:

const getElem = document.querySelector('#custom_field_123")

Now I don't see any of that with the garden markup. How are we to develop custom functionality without unique IDs?