In desktop automation mode, you can still use all Selenium IDE commands for automating the browser! You can have both in one macro.
Or, if you want to solve this purely with desktop automation (e. g. for Internet Explorer or Safari), use XClick and XType, like here: Copy Image from url and Paste into Whatsapp Web - #2 by ulrich
After doing XType | ${KEY_CTRL+KEY_V} the value is in the !clipboard variable.
See also UI Vision Desktop ( Select text, copy, and paste) from word document - #4 by Plankton