As far as I can tell from looking around, with an AMD card and Freesync enabled it works fine at 75Hz. I have read somewhere that it will show as 75Hz with Nvidia cards, but won't actually be running at 75Hz (34UM88 that is).
If windows is reporting 75 he ? How else can we test ?
However, you can do this effectively only with AMD card, because even though it theoretically works with Nvidia cards, there is phenomenon called frame skipping that will cause all your system and monitoring tools report 75 Hz and FPS while you will see only 60 FPS effectively and those additional 15 frames are going to be skipped.