How to explain to my users that only one form needs completed

I have a form that allows a user to enter a product link, previously they had three fields: a brand and URL were required, and an optional notes field.

They get the URL from another system and after observing user behaviour I found that they were almost always entering an ID from the other system in the notes field.

To make this process less prone to human error, I constructed an API that allowed the form to obtain a list of products from the other system, the user selects the product from a list and it automatically fetches the correct URL, correct ID which it adds to the notes field, and allows for some additional functionality using extra information it can gather from the API.

I have added the form to use this method above the original form, and explained to the users that it is preferable to use this one where possible, and to use the old form if custom data is being entered that does not fit the model employed by the new form.

I thought this was simple enough. However, I have witnessed users getting confused and doing all sorts of silly things. Entering a URL the old way then selecting a product, clicking the wrong 'create' button etc.

How can I make it more obvious that these forms are exclusive, only to use one or the other?

I have considered hiding the old form (with the 3 fields) unless a button is clicked to show it, or maybe use tab controls, but I feel these options make the forms less accessible.

Edit: I should clarify, these forms are for internal use only, not public, and will be used by experienced administration staff