Currently autofill works for username/email and password only. Proton pass already supports custom fields. Give ability to autofill those custom fields.

Bitwarden supports this by correlating custome filed name to id, name, aria-label, or placeholder... of the html element. Their browser extension supports automatically copying name value from webpage.

https://bitwarden.com/help/custom-fields/

https://bitwarden.com/help/auto-fill-custom-fields/

Details mentioned Here custom field and ots autofill support for more advanced usage.