Why am I getting the 'Opportunity creation failed. Required Fields missing: [custom_field_name]' error when trying to create an opportunity in Salesforce from the Freshdesk widget?

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Mandatory custom fields in Salesforce Opportunities are not supported, for now, in the Salesforce app for Freshdesk. 

If you try to create an Opportunity from the Freshdesk widget when a mandatory custom field is required, the widget will show you this error message.