I'm new to SharePoint programming or modify form. I have a tenant with SharePoint Online and I created a list with calendar view: I'd like to modify the form when a user insert an event. I tried to load the site with SharePoint Designer 2013 and tried to create a new form but obtained the error dialog with message "Could not save the list changes to the server." After this, I tried to open the NewForm.aspx to see the code and maybe change it directly, but I can't see all the code: it seems that the part where there is the javascript is missing and there is a tag of missing assembly (sorry in the screenshot is in italian)
So I tried with Visual Studio, but when I try to create a solution I obtain the error in the screeshot
So the question is: I'm a beginner and I want to start with modify the NewForm.aspx so I can cancel some field to test: how to procede? I found many links and references but it seems that SharePoint Online have different behaviour versus the old and on premise editions.
Any idea in what mode I can procede?