Moving types and slices from an account with Free Plan to a separate account with Medium Plan

I'd like to move my Custom Types and Slice definitions from a free repository (that's been created for development purposes, during the website development process) to a separate account (that belongs to a client) with a Medium Plan, that supports importing and exporting.
When importing the compressed 'customtype' JSON schemas from the project, the import tool requires maskId and there is no mention of such property in the documentation.
I do not wish to keep content from the Development Website, as it is all placeholder data. I want to move the Custom Types and Slices to the new repository for the Content Creators to use.

I have all of the definitions on my machine stored locally on my machine.
I have admin privileges in the new repository.

What I tried
I compressed index.json from the folder customtypes and use it in the Import/Export section. This resulted in an error, that required a type property in the JSON, which I then added by hand.
After adding the type it changed to an error that said that 'type' must correspond to a maskId in this repository.

Hi @abrymora
Thanks for reaching out.
In fact, the Import/Export feature is not meant to import Custom types and doesn't work for that purpose.

To achieve what you want, you either should go to each Custom Type and copy the JSON in the JSON editor, or you can use the Custom type API to retrieve them as explained in the document.

Please let us know if you need any further assistance,