how to get contact ID for custom function

how to get contact ID for custom function

I am going to create a button that call API from our app (3rd api). I want to pass contact Id and user email as params. I define my  custom function as shown :
  2. contDetails = zoho.crm.getRecordById("Contacts",input.contactId.toLong());
  3. paramsToPass = Map();
  4. paramsToPass.put("User_email",ifnull(contDetails.get("User_Email"),""));
  5. paramsToPass.put("Contact_id",ifnull(contDetails.get("id"),""));
  6. info paramsToPass;
  7. return paramsToPass;
when I save and execute , there is a pop up and ask me to put contactId. Is there any way that we automate dynamic get id from a contact?