TypeScript: Define record where keys must be belong to union

type CompanyTypeCode = 'ltd' | 'plc' | 'sole-trader'

type EmailTemplates = {
  [Code in CompanyTypeCode]?: InvitationEmails
}

// Because we made the key of EmailTemplates optional, we don't have to define every value of the CompanyTypeCode union as a key
const emailTemplates: EmailTemplates = {
  ltd: LimitedCompanyEmail,
  plc: PublicCompanyEmail,
}