how do I get rid of index.php in link to the forums?

Have you used the .htaccess to remove index.php from the url?

https://docs.expressionengine.com/latest/installation/best-practices.html#removing-indexphp-from-your-urls

(not sure if it’s the same for forums, though)

for a link once you have your forum URL, you could add:

<a href="http://{path=forum}">Forums</a>

where you want the link to display

https://docs.expressionengine.com/latest/templates/globals/path.html#path-variables

(edit: for some reason ‘http://’ is being added to my code snippet, but don’t put that in if you use {path=""} )

(editedit: ok, there is an issue when adding expressionengine code with markdown, the example link will need some tweaking to actually work - link provided to examples)