The original poster can add a tag to the 1st post. Underneath the Topic Title, is a field for "Topic Tags". Just add the tags there. As far as I know, only the original poster can add tags to the 1st post. The original poster can later edit the original post and add additional tags.
To add tags, go the your 1st post, click on edit.
Underneath the Topic Title is a field for "Topic Tags". Just add the tags there.