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I would strongly advice against that. It's just wrong.
But anyway this post on Stack Overflow has some options you can try putting on the same level as extended_valid_elements: http://stackoverflow.com/questions/3554338/insert-line-break-instead-of-p-in-tinymce
Yes, and the value of EditorInitConfigurationOptions is a string of javascript. The values of your plugin will merge with Epi's default TinyMCE configuration and change the way an editor instance that has that plugin activated behaves.
Pseudo-code with the example from your post and the link in first reply:
EditorInitConfigurationOptions = @"{ extended_valid_elements: 'iframe[src|frameborder=0|alt|title|width|height|align|name|style|id|scrolling]', force_p_newlines : false, force_br_newlines : true, convert_newlines_to_brs : false, remove_linebreaks : true }"
I'm trying to enable soft retun, meaning soft line breaks.
When make a regular return following code is generated inside TinyMCE
A soft return (shift + return) return
I have a MVC project with EPiServer CMS 9 installed. Within the web project I have this class, which I believe deals with the customization of TinyMCE