Thank you for your interest in Syncfusion products.
Regarding Mail merge with Ask & Fill-In Fields:
Currently DocIO performs mail merge operations with the Merge fields only and doesn’t have support for Ask and Fill-In fields during mail merge. We have logged this as a feature request in our database. We will implement this feature in any of our upcoming releases. We usually have an interval of at least three months between releases. The feature implementation would also greatly depend on the factors such as product design, code compatibility and complexity. We will update you when this feature has been implemented.
As Workaround kindly have all the fields as Merge fields in your template document in order to perform mail merge using DocIO.
Regarding the replacement of Merge field with Text:
We have prepared a sample to illustrate how to replace a merge field with custom text. Please find the attached sample and let us know if this helps you.
Please let us know if you have any other questions.