Is there way to prepend 'http://' to URL if it's not there while adding URL with Insert Link in TinyMCE?
For that you would need to copy the tinymce Insert Link plugin, rename it, add the necessary code to it (the "http"-Adding) and use it as your own plugin.
EDIT: Ok, here is an example (using jQuery):
// You might need to change the event and/or tha handling
// but this should give you a guess at what needs to be done
setup : function(ed)
{
ed.onClick.add(function(ed, evt)
{
$(ed.getBody()).find('a').each(function(index, element){
if (!$(this).attr('href').search('http')){
$(this).attr('href', 'http://' + $(this).attr('href'));
}
});
});
},