You can disable mMaya from booting at startup by editing the userSetup.py file in your maya folder.
By default, this is located in : C:\Users\[UserName]\Documents\maya\2018\scripts\userSetup.py.
You can comment out the code responsible for booting mMaya by adding a ‘#’ character at the beginning of the line, which should end up looking like the line below:
#import sys; import maya.cmds as cmds;sys.path.append('C:/Users/[UserName]/Documents/maya/scripts\mMaya'); cmds.evalDeferred('import mMaya_rootEnv; mMaya_rootEnv.init()')
To boot mMaya, you can then copy/paste the code that we commented out (without the ‘#’) in Maya’s script editor (in a python tab) and hit control+enter to execute it.
You can also create a shelf button to launch mMaya by pasting the code in the script editor and drag/drop it over a shelf. A popup will ask you in what language the code should be interpreted, which should be “Python”.
After rebooting Maya, you will be able to boot mMaya by clicking on the shelf button.