MX provides the functionality to create new fields using the application extensibility framework. A restart of the application is needed to have those changes take effect.
The files which allow changing the UI are in the content management tab and would need to be downloaded, modified, and uploaded again in order to succeed with the change. Please be aware that no syntax checks will occur while the file is uploaded – a faulty code line could stop the application of work.
It is therefore recommended consulting with your Customer Success Manager and/or your implementation team.