How to add Source Code button to WYSIWYG

Hello, I’ve upgraded an older site that utilized the source code button. I had previously just customized the buttons that appeared via a simple script in functions. This no longer works for ‘code’ and it seems as though WP has done away with the source code component of TinyMCE? I have tried to use TinyMCE Advanced but the source code is also not working in 4.6 and I’ve been unable to get any support on that front. Since I really just need this one button, I’m wondering if there is a simple way to add this back in as it is important at this time. Thanks.