Here's a guide from our Wiki on how to use your Fediverse account to post, reply, and follow, and generally interact with the Fedi Forum via your own account https://thefedi.wiki/forum

It is possible to use your fediverse account to create a new topic. This could be, for example, using your own Mastodon account, or GoToSocial, or Misskey, etc. To achieve this, mention the fediverse address of the category in your post in your own server post. Example: @general-disussion@thefedi.forum All posts are public, including your fediverse address. (The addresses appear in the description for each category)

In order to reply to a topic/post you can register on the forum. Alternatively it is possible to use your fediverse account to respond. This could be, for example, using your own Mastodon account, or GoToSocial, or Misskey, etc. To achieve this: Find the post you wish to reply to in the forum. Locate the post number on the right, at the top of the post content, or one of the replies. Right-click on the # number in your browser and select 'Copy Link Address' or equivalent. Return to your own fediverse account/software. Paste the copied address into the search bar. You can then click on the post and reply. Be aware, all posts to the topic are public!