please , can explain it in step by step.
To start with you’d need JWalk or a similar package, then you can use the package to modify your screens to use a GUI interface with your watermark embedded. You then use the packages client software or a web browser instead of client access.
(Unfortunately these sort of packages don’t normally come cheap, so it would be a pretty expensive watermark.)
iSeries Access for Web could be a cheaper than Jwalk solution but it’s going to give you that function without some serious mods. You could do this sort of layover with AS400 printing but if you’re not already using AFP’s then you may not want the headache.
If I were you I’d just add a disclaimer to the AS400 Login screen if you’re trying to add some legal formality to people viewing/access the AS400.