Always check the server's "Allowed Modifications" page. Using Auto Place Mod 1.8.9 on a competitive minigame server is considered cheating by the majority of the community and administration.
After launching Forge, a mods folder is created inside your Minecraft directory. You can find it by: auto place mod 1.8.9
(three modes)
For legitimate fast placement, learn advanced bridging techniques – they are more satisfying and safe. Always check the server's "Allowed Modifications" page