In order to do that, you will need to write (or find) some code in another programming language, such as C, making use of the published format of .MAT files, and have that code create the internal representation of the structure field using the prohibitted name.
Once you have the resulting .mat file, you will need to load it with an older version of MATLAB. I do not remember exactly which release started to refuse to load .mat files with invalid field names, but what is coming to my mind is that it might have been roughly R2018a. So if, for example, you were to use R2012a or so, you would be able to get MATLAB to read the .mat file that the outside program had created with invalid field names.
You might want to look in the File Exchange as there is a contribution there that is intended to repair files that had invalid field names -- because of course if you have code to repair an invalid field name in a .mat, then that implies that with a small change you could instead have the code deliberately break the .mat .
... In case it has no been clear yet, what I am saying is that MATLAB does not permit field names of that form to be created inside MATLAB, and these days will not even load files that have such names, but if you really need to (for some reason!) you could create them for use in old enough versions of MATLAB.