hey everyone I hope I can help you fix your problem when u try to get on 7.3.5 ptr,
First of all you wanna download
Windows https://nofile.io/f/BWm1MJJ9Nr2/firestorm_735_ptr_patch.zip
Or
Mac https://nofile.io/f/P9uuTfwB5Bc/firestorm_735_ptr_patch_mac.rar
After that Download World of Warcraft : https://eu.battle.net/account/download/
Make a folder called PTR 7.3.5
When you finish installing battle.net, and you see the playable bottom you want to exit battle.net,
Now go to battle.net folder which you can find it in program files x86 depend where you saved it,
Extract the files you downloaded from the first links I showen, Open wow,ptr64 and that's all.
Last step : enjoy