When mapping fields, you can configure additional text to be appended during sync to the same field value in the destination org for fields of type String and LongString. Text including URLs can be added before or after the field value, or both.
Before saving a field mapping, change the value of the Custom Format field to Yes.
Add appended text to the field Custom Field Value. Text should be added in the following format: "appendedText1{value}appendText2". During sync {value} is replaced by the actual field value in the destination application. For example, to append a URL to the value of the field, you might specify the custom field value as something like "https://yourdomain.my.salesforce.com/{value}/abc".