Option to prepopulate Node Author

The reason it does not work with node:uid is that,
perepopulate field values from query string only works when the field has no default value.
"Entity owner" field usually has a default value (current user), because of the BaseFieldDefinition::setDefaultValueCallback().

Non empty fields can't be overridden from query strings.
See Populate::populateForm().

When I temporally remove that condition and I use this query string ?edit[uid][widget][0][target_id]=42 then it looks good in the node create form. I did not press the "Save" button.

I hope this information helps you to find a custom solution/workaround.

In my use case, I would like to make it easier for administrators to create a node for somebody else. They already have access to the "Authored by" field, just make sure they don't forget to change it.