wpkg --output-dir ...
The --output-dir option is used along the --build command to define the destination directory of the generated package. By default the new package is saved in the current directory.
If the specified directory doesn't exist yet, then it is created. If a file of the same name (but not a directory) already exists then the saving of the package fails. You also need write permission in that directory, obviously.
This option is particularly useful if you have a reposity directory where you save all your packages although in that case you may want to use the --output-repository-dir option instead since that other option also takes the Distribution field in account. Note also that the --output-repository-dir option has precedence over the --output-dir option.