Ensure you have run this version at least once in your Minecraft Launcher.
Most Wurst versions for 1.12.2 use a standalone installer. Open the downloaded file and follow the prompts to create a new profile in your launcher.
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 Download Wurst v6.35 MC1.12 OF
Includes built-in support for OptiFine (OF), which can be added manually to the mods folder or is sometimes bundled in the installer depending on the specific "OF" edition. Installation Steps
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 Ensure you have run this version at least
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
Only download Minecraft clients from official sources like WurstClient.net. Downloading from third-party "re-upload" sites or YouTube description links can expose your computer to malware or account theft. How to Install Minecraft Hacked Client for 1.12.2 - Wurst If your version did not come with OptiFine
Get the .jar file for v6.35 from the link above.