I need to run "xmousepasteblock" automatically on login but on current session.
When I run this command via terminal or ALT + F2 it works correctly, but if I run it via cron for example:
@reboot xmousepasteblock it doesn't work. I have tried via the integrated Autostart manager in KDE Neon but doesn't work as well (or I didn't do it correctly).

Running "X Window System" clients via
"@reboot" crontab job-entries is wrong.

Remove it from Your crontabs wherever
You entered it.

I have removed it from crontab but how can I make it to start automatically on startup?

Has to be started after PlasmaDesktop login:

SystemSettings=>StartupAndShutdown=>Autostart