We plan to include the ability for the Work Order ID field to be pre-populated based on expression in our roadmap; we're working with other areas of the ArcGIS platform on this.
One technique I have seen used for sites who need this ability is to use a Python script to populate the Work Order ID field at regular intervals (i.e. every 5 mins) for assignments after they are created.
It's not assigned to release at this time, but I am hoping we can get to this in one of our next few releases.
I've added your request and info to the issue for tracking purposes.