You can use the 123FormBuilder – Zoho CRM integration to set up Zoho CRM custom fields in a few simple steps.
First, create the field in your Zoho CRM account. You will need a paid subscription with them to be able to add your own custom fields.
Now, in your 123FormBuilder account, in Settings → Applications, assuming that your integration with Zoho CRM is already enabled, click Customize in the application box.
The lightbox that shows up is where you need to match 123FormBuilder fields with Zoho CRM fields. Scroll down and tick the checkbox I want to use custom fields. 100 new rows will be added to the list. Use them for your custom fields.
Type the exact name of your Zoho CRM custom fields from your account on the left and match them with the fields in your form.