Want to change the default value for "Planned For" on a task to the current iteration.
One answer
The standard is unassigned as far as I can tell. I would suggest to check your customization and adjust it accordingly.
I am not sure how a default value provider could be able to even find an iteration since at creation time there is almost no information. I would assume that finding the current iteration requires Java Customization/a Java based calculated value, if it is possible at all.
If you talk about creating work items in a plan, that is a completely different matter.
PS: It is unclear if the answer can be accurate as the information given and the tagging leaves a lot to be desired.