Middle mouse paste is built into the [Scintilla](https://www.scintilla.org/index.html) editing widget Geany uses which does not have an option.

Note that the desired functionality sylphenix requested is NOT necessarily 100% synonymous with the unix stackexchange link, as the latterrefers to all programs. For instance, quick copy/pasting may still be useful on a terminal such as KDE konsole, whereas it may be more annoying in editors. So I think the question should be whether the time investment for this option is worth it or not. I think it may be useful under a mouse/scroll settings/options.