Apply weekly backup plan to the servlet using API

Hello

I have a question about how to apply weekly backup plan to the servlets created automatically

 const droplet = await client.droplets.create({
            name: domain,
            region: 'ams3',
            size: 's-1vcpu-1gb',
            image: snapshot_id,
            ssh_keys: [await this.get_ssh_key()],
            backups: true,
            plan: 'weekly',
            ipv6: false,
            private_networking: true,
            volumes: null,
            tags: ['test'],
            user_data
        });
        

Field ‘plan’ doesn work in this context

Thank you …