You will probably not get it. Read here: https://community.ptc.com/t5/PTC-Mathcad-Tips/Official-PTC-Mathcad-15-and-Prime-1-6-End-of-Sale-Anno...
Instead you can try a trial for Mathcad Prime 8. See here for installation: https://community.ptc.com/t5/PTC-Mathcad-Installation/All-About-Mathcad-Software-Installation/m-p/70...
Success!
Luc