Hello Leonid,
If you want to use fields created by the key user in the SDK you need to map them via the item "References to Customer-Specific Fields".
Here you can specify which fields shall be available in the SDK.
Be careful: If you deploy the solution to the prod / test system you need to create those fields with the same name, type, ... manually in advance before deploying the solution.
As fields created by the key user are tenant-specific they are not part of a tenant copy neither are they deleted if you deploy a solution (as Alessandro mentioned).
bye,
Horst