Hi - the cleanest way would be for that application to set the MIME hint x-kde-passwordManagerHint : How to uninstall Clipboard? Removing doesn't work from Discover. This showed my password! - #44 by ngraham
An example of that being implemented - in this case, in KWalletManager - is located here: Add x-kde-passwordManagerHint when copy from kwalletmanager (!46) · Merge requests · Utilities / KWalletManager · GitLab