adding apps to the multiwindow bar

[almaach]

Hi
I've looked through the thread and this variant of the "how to add apps to the multiwindow bar" question doesn't seem to have come up.
I noticed a few apps seemed to have added themselves to the multiwindow bar, without intervention from me. Very handy they were too (in particular Dashlane - brilliant having that as a multiwindow app)
However they have now removed themselves from the sidebar; Dashlane removal was probably due an update in the Dashlane app
How do I get them back on the multiwindow bar?

Thanks!