can't add a from mailadress from a shared mailbox

Hi all

Normally i can add a mailadress to the from field in outlook from a shared mailbox where the delegate rights are set correctly. With a new setup (outlook 2019 - o365) i can't anymore, ik can click on the from button and select the mailbox by clicking from again, but i can't seem to add the mailadress to the list as i used to? When i click the from button i get a new window that has 2 lines, it used to be just the one. Is this a problem with the latest outlook version?

Any ideas?

Thanks, best regards

M van Vugt