I'm not sure I understand what you're asking. It's only going to assign one of those two values. There's a 15% chance it assigns the value "Seed" and an 85% chance it assigns the value "Serial".

Then the By Percentage pickoption isn't going to be useful to you. The question is 15% of what?
The simplest way is probably to set this up in an arrivals table. Like if you have 100 items arriving make 2 entries. One with 15 items that get the Seed label and another with 85 items that get the Serial label.
Alternatively you could write a code snippet that keeps track of how many items have arrived and if the number goes above your 15% number then you switch. Something like:
string labelValue = "Seed"; if (current.NumArrivals > 15) labelValue = "Serial"; item.Type = labelValue; current.NumArrivals++;
I've attached a sample model with these two approaches.

Thanks for the help.