At the top-right of the page, click on Settings.

In the My Settings section on the left, go to General Settings.

Scroll down to the "Miscellaneous Options".

For the "Message Editor Interface:" select the Standard Editor.

At the bottom of the page, click Save Changes.

Your default editor will now be "Source mode".

This could be made the forum default, but it would only apply to new users when they first join.
Each existing user will have to change the setting themself.
Depending on the forum permissions, this may not be available for everyone.

Please report if it is, or is not available to you.