It is always possible to copy and paste.
Please give more context. Are you talking about the forum or an external program? Do you want to copy an image of a formula or the LaTeX text that generates it?
When one clicks on "Reply to Thread", the forum opens a built-in mini-editor where one types the response. This editor offers typical basic functionality, in particular, keyboard shortcuts. It understands Home, End, Shift-arrows to select text, Ctrl-C to copy, Ctrl-V to paste, etc. You can also select Edit | Copy from the menu in your browser after selecting some text.