I think you are pretty close to the solution. I tried making it work myself and it was not too difficult, perhaps more complex behavior can be designed, but I just made a basic automatic pickup.
You need to issue the command of the Inventory ability type that the unit probably has. The ability command ID for 'Take' is 2, so the Ability Command field of the Issue Order effect should be set like so. If you'd like to know different commands and their IDs, you can take a look at the ability Commands field and count from the first being ID 0.
I hope this gets you up to speed. Another useful note is to exclude 'Hidden' from the search area, otherwise it will include also the items already in units inventory. I guess best is to use an Iterate Inventory effect to drop specific items, but I haven't used this type yet, so I can't say for sure.
Anyhow, if you require any more assistance I'd be happy to help. Take care.