There are 2 ways to do it. One is still a mystery to me as to how it works, the other uses more bytes but is easier to understand. Type 1:
There's no setflag command in the script, but it appears to set one anyway, basing it off the person ID in AdvanceMap, but I believe only certain flags work. 01C0 and up can be used, but I don't know where these end and the regular flags begin. Just set the flag to Person ID of the Pokeball.
Now, the standard script, uses many more bytes but makes sense. Set whatever flag you used to the Pokeball's person ID.
If you aren't at that point of scripting yet, the var stores the number of the item you want, and the commands later using 0x4000 read that var. The bufferitem stores the value of 0x4000 as an item name to buffer \v\h02. So, if you set 0x4000 to 0xD, the script will add a potion and display "(name) found one POTION!" I did it this way so all you have to do is copy/paste the script and change the value of a var and flag to use other items. Set the pokeball's person ID to whatever flag you used (200 in this case) to make it vanish after the script.