A custom amount that changes each time is very difficult. If the amounts are more consistent, I do that for priming and stuff on mine.
The way you do that is set up a profile of type DOSE with the amount you want, speed I usually set 250ml/min so it happens fast and interval set to 1. Let's call this primeDOS as an example
Create a virtual outlet with the following code. Change MMM:SS to an value longer than the dose would take (ex larger than 2 minutes for a 500ml dose). I'm calling this prime.
Set ON
When ON > MMM:SS Then ON
In the advanced tab of the DOS programming add
If Output prime = ON Then PrimeDOS