The download link of mathtype6.8 for windows is below
link: https://pan.baidu.com/s/16SafvtBptPNQMunzMv3YMg
pwd: e9jf
Download it and install it to the folder you like, during your installing progress, choose the same options as below
Filling in the box with *, you can get the product key from the link : https://www.smartserials.com/serials/Mathtype_6.8_33509.htm
Before that, you should verify you're human.