How to add shortcut/streamline of subtools?

How to add shortcut to the button/subtool? Example, I am in the move option and I would like to relax my mesh instead of going into the tool window and pressing the relax, I would like if I could just press shift and it transform into relax and when I release shift it goes back to move.

I can't see a possible way for you paticualar case. Because when some visual tool is on, short cut tools are prohibited as the visual tool may have it's own internal hotkeys which may conflict with the shortcut keys you want to apply.

My suggestion is you can set up two streamline tools. One for the tweak-move, there is a basic streamline tool 'Subobject Tools > Brush_Move' for this. And the other streamline tool for the relax function. I know some people assign 'Relax' to 'On WMB Up' and 'Undo' to 'On WMB Down'.

Another suggestion is you can just setup one streamline tool. Assign 'Brush_Move' to 'LMB Tool' and 'Brush_Relax' to 'RMB Tool'.

If you don't know how to set up streamline tools, there is a tutorial in the help file. Help > Tutorial > Set Up A StreamLine Tool.

Thanks for the reply, I have started a Feature Request for this kind of shortcut but I don't it's possible because no one replied. I have recently bought the software and I'm pretty happy with it. I think with your reply one of my worries about the software may have been lifted. Thanks again.(Sorry if my English is bad)