Hi, @theirishkiwi.
This Support Guide is the first port of call to debug any forms issues. There are also many other Support Guides for forms - you can find them here: #Netlify-support:support-guides
We also recommend trying to search the forums or look at topics tagged Netlify forms if you haven’t already - it’s likely your question was already asked by someone else!
We do cover this issue in that support guide.
This the the summary of the issue. There is a requirement that all fields which could possibly be sent be defined in an HTML version of that form somewhere. This field isn’t in the HTML so it doesn’t work.
It doesn’t have to be in the same HTML file but all possible fields must be defined in HTML somewhere in the site’s files before they will work.
Because this form doesn’t have that field defined in the HTML, the backend is not configured to accept it as an input. It won’t work until it exists in an HTML version of the form which, again, can be a separate file only used to create the backend form handler and not used for anything else.
If there are other questions or concerns, please let us know.