If you are new to SharePoint Online development, the first task as a SharePoint Developer, would be to create a HTML form and update’s an item to SharePoint List. In this blog we will create a very basic HTML and create an item using SharePoint JSOM and add the form to the page using Content editor web-part.
For sample code and more details see: SharePoint Online create a form to create list item(JSOM)
DC~Hippu 