There are lots of SEO techniques, you will find while searching on Google. Including all those common practices that are mostly followed by all SEO professionals. There is one difference in performing all activities and that is quality of work. Always choose quality over quantity.
Forum posting sites are for sharing your knowledge to others related to your expertise. Form platforms should be used for sharing knowledge. For example this forum is SEO knowledge sharing platform, same forums will be there in your niche,
To find relevant forum platform and share and get knowledge related to your business niche should be a purpose to register in forum sites.
Here is the trick to find forums in Google