Hover over the message or file you want to send, and click the arrow in the top right.
Add the people or chats you want to forward the message to, write an additional comment for context, and press Send. The forwarded message will be indicated by a green line on the left of the text.
(Note that in chat rooms, the room owner has the option to disable the ability to forward messages and files. This setting is found in Chat Details. Navigate to the i in the upper right hand corner, click Options, click Chat Settings then look under Member Permissions or Chat Settings. If you are able to copy messages, it will say Members can copy room messages.)