How to get custom field change in node JS

Hi
I am trying to update a field in salesforce and its a custom field in FD I am only able to get that field when I am using cf_merchant_id_3202350 but field name is cf_merchant_id please tell me why and how to get this number

if (args[‘data’][‘ticket’][‘group_id’] == ‘1082000658705’) {

        try {
          const response = await fetch('https://razorpay--produat.sandbox.my.salesforce.com/services/oauth2/token', options);
          const data = await response.json();
          console.log(data);
          let access_token = 'Bearer ' + data.access_token;
          console.log(data.access_token);
          await updateSalesforceRecord(access_token, args);
        } catch (error) {
          console.log('error at ', error);
        }
      async function updateSalesforceRecord(access_token, args) {
        try {
          console.log('ard' + args['data']['ticket']['id']);
          let options = {
            method: 'PATCH',
            headers: {
              Authorization: access_token,
              'Content-Type': 'application/json;charset=UTF-8'
            },
            body: JSON.stringify({
              Status__c: args['data']['ticket']['status'],
              MID__c: args['data']['ticket']['custom_fields']['cf_merchant_id'],
            })
          };
          console.log(`https://razorpay--produat.sandbox.my.salesforce.com/services/data/v55.0/sobjects/Tech_Support_Ticket__c/FreshDesk_Ticket_Number__c/${args['data']['ticket']['id']}`);
          console.log(options);
          const updateResponse = await fetch(`https://razorpay--produat.sandbox.my.salesforce.com/services/data/v58.0/sobjects/Tech_Support_Ticket__c/FreshDesk_Ticket_Number__c/${args['data']['ticket']['id']}`, options);
          console.log(updateResponse);
          const updateData = await updateResponse.json();
          console.log('Update Response', updateData);
        } catch (error) {
          console.log('Error updating Salesforce record', error);
        }
      }
  }