Hi anonymous user
If you want to use external website with custom form, we can using HTML code and jQuery code with REST API, you could refer to this article:
https://learn.microsoft.com/en-us/answers/questions/572293/create-item-in-sharepoint-2019-list-from-external.html
https://sharepoint.stackexchange.com/questions/270361/how-we-can-read-external-sharepoint-list-data-using-rest-in-sharepoint-2016-2019
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