Although I'm always interested to see where tgui is used, I'm only asking for a simple explanation on why you need to do this. I'm not going to add a function to edit box if it has no use, but if you can give me a use case then I will add such a select function. The function would be editBox->select(begin index, end index) rather than the setTextSelect that you suggest, but it can do the same. I currently just can't come up with any situation in which you would need such function.
I only use windows when I have to, I'm normally always on linux.