If your version did not come with OptiFine bundled, download the 1.12.2 version of OptiFine and place it in the .minecraft/libraries or mods folder created by Wurst. Safety Warning
Get the .jar file for v6.35 from the link above. Download Wurst v6.35 MC1.12 OF
To download (with OptiFine compatibility), you should use the official Wurst Client website to ensure you are getting a safe and authentic version. Official Download Link If your version did not come with OptiFine
Ensure you have run this version at least once in your Minecraft Launcher. Download Wurst v6.35 MC1.12 OF
You can find the specific archive for the 1.12.2 version on the official Wurst Client Download Page. Wurst v6.35 Minecraft Version: 1.12.2