I'm able to click-and-drag a box representing units or items from the left-hand side of the trade screen to the right hand side of the trade screen. However, with Prisoners, that only works if I put their box in my Troops section - if I put them under Prisoners section (not Prisoner header, that one works), they just stay where they were. I think this is an oversight and should be adjusted so that putting Prisoners anywhere on the right-hand side of the screen should register as me wanting to imprison them.
Supplementary video demonstrating the issue.
Supplementary video demonstrating the issue.