Got it:
$filterfields = [
'id' => 'ID',
'firstname' => 'First Name',
'lastname' => 'Last Name',
'companyname' => 'Company Name',
'email' => 'Email',
'address1' => 'Address 1',
'address2' => 'Address 2',
'city' => 'City',
'state' => 'State',
'postcode' => 'Postcode',
'country' => 'Country',
'phonenumber' => 'Phone Number',
'currency' => 'Currency',
'groupid' => 'Client Group ID',
'credit' => 'Credit',
'datecreated' => 'Creation Date',
'notes' => 'Notes',
'status' => 'Status',
'emailoptout'=>'Email Optout',
'marketing_emails_opt_in'=>'Email OptIN'
];