Hello,

For each custom field you create, you have a custom label id and input id generated.

This way :
label : jform_custom_fields__[YOUR SLUG]-lbl
input : jform_custom_fields__[YOUR SLUG]

You can use a code inspector to get the id for each element (and all classes parent).
eg. with Firefox, a right click on the page, and click on "Inspect element" will open the developer tools

Best regards,
Cyril