With KaTeX you can place mathematical function and all other TeX related features on your website with in Kunena forum posts.
A small demo you can see at: http://demo.schultschik.com/jatex
For KaTeX the LaTeX Code have to be between [katex]LATEXCODE[/katex] or $...$. Since v1.4.0 $...$ works as well.
You have trouble using the extension? Write an issue at github or see the support website which other possibilities you have.