cpList = List();
firstName = input.Contact_Name.first_name;
lastName = input.Contact_Name.last_name;
contactName = firstName + " " + lastName;
contactMap = Map();
contactMap.put("contact_name",contactName);
contactMap.put("company_name",input.Company_Name);
contactMap.put("website",input.Website);
contactMap.put("twitter",input.Twitter);
/////////
//contactMap.put("skype_name",input.Skype_Name);
billingMap = Map();
billingMap.put("zip",input.Billing_Address.postal_Code);
billingMap.put("country",input.Billing_Address.country);
billingMap.put("address",input.Billing_Address.address_line_1);
billingMap.put("street2",input.Billing_Address.address_line_2);
billingMap.put("city",input.Billing_Address.district_city);
billingMap.put("state",input.Billing_Address.state_province);
contactP = Map();
contactP.put("salutation",input.Contact_Name.prefix);
contactP.put("first_name",firstName);
contactP.put("last_name",lastName);
contactP.put("designation",input.Designation);
contactP.put("department",input.Department);
contactP.put("contact_salutation","Mr/Mrs");
contactP.put("mobile",input.Mobile_Phone);
contactP.put("phone",input.Phone_Number);
contactP.put("email",input.Email);
//Skype does not work here or on the contactMap
//contactP.put("skype_name",input.Skype_Name);
contactP.put("is_primary_contact",true);
contactMap.put("billing_address",billingMap);
// Custom Fields
cpList.add(contactP);
cFields = List();
cFields.add({"index":"1","value":input.Registration_Number});
cFields.add({"index":"2","value":input.Industry});
cFields.add({"index":"3","value":input.VAT});
//next doesn’t work
//cFields.add({"index":"4","value":input.Legal_Form});
cFields.add({"index":"5","value":input.Tax_Dossier_No});
cFields.add({"index":"6","value":input.Legal_Rep_SSN_No});
cFields.add({"index":"7","value":input.Balance_Sheet.toDecimal()});
cFields.add({"index":"8","value":input.Annual_Turnover.toDecimal()});

These are the fields :

//cFields.add({"index":"9","value":input.Legal_Document});
//cFields.add({"index":"10","value":input.Accounts_Document});
//cFields.add({"index":"11","value":input.Tax_Document});

contactMap.put("custom_fields",cFields);
contactMap.put("contact_persons",cpList);
params = Map();
params.put("JSONString",contactMap);
createContact = invokeurl
[
    url :"https://books.zoho.eu/api/v3/contacts?organization_id=..."
    type :POST
    parameters:params
    connection:"..."
];