Copy the text and/or links you want in Mail. Paste it into the text editing box in the forum.
In the email's body, tap-hold for the popup menu, then use Select or Select All and drag the blue handles to adjust the selection (if needed). In the Forum, tap-hold for the menu, and choose Paste.
Or you can open the site in Safari by tapping the link in Mail. In Safari tap on the URL address, then choose copy. Go to the forum and paste as before. Or you can use the Add Link tool. It's the three little circles, one of them green.
Before you do, make sure you are familiar with the forum's rules about posting outside links.
For YouTube videos, what works best is to simply paste the link. It should show the video in the post automatically. It needs to be a direct link to the video.