In 2015, Moodle introduced a new editor which makes contributing to forums with an iPad much more straight forward than it was previously. However, the following may be useful if you want to write your post in a word processor or notepad first and then copy and paste the text into the forum post editor.
Copying and pasting text
Copying and pasting text using an iPad is possible. Guidance is available in the Macworld article: How to copy and paste text in iOS.
Uploading a file as an attachment to a forum post
You can upload attachments to your forum posts using the same work-around described in Using an iPad to upload files.