[britechguy]

At the time of this writing, there is no "Mark Solved" button or function.

Use the triple dot menu at the very top of the topic, which is only visible to the Original Poster (OP) that started a topic:

And, once activated, choose Edit thread.

You'll then be presented with a dialog to edit your Title. Change 'No Prefix' to [Solved]:

Finally, activate that Save button to save the change in the topic title.