How can I assign WorkItems to a particular user by default?

No, there is no way to automatically assign a work item to a person.
Selecting a "Filed Against" value (i.e. a "Component") associates a work item with a team but not with a person. The assumption is that someone within the team does the "bug triage" and assigns the work item to a team member.
The person who files the work item cannot know the team or person who is responsible for the item.

I submitted workitem 78526 for this feature.
In particular, it's asking for the ability to assign a "default owner"
to a category, and initialize a new work item assigned to that category
with that user as the owner.

Cheers,
Geoff

stephen_kruger wrote:

Is there a way to allow people to simply select the component to file
a workitem against, and have it be assigned to a person responsible
for that component?
It's not always obvious across large teams who owns which component,
and we are getting lots of WorkItems being created with no
assignment.

thanks
Steve