ProtectedForForms

public bool ProtectedForForms { get; set; }

Document doc = new Document();

DocumentBuilder builder = new DocumentBuilder(doc);
builder.Writeln("Section 1. Hello world!");
builder.InsertBreak(BreakType.SectionBreakNewPage);

builder.Writeln("Section 2. Hello again!");
builder.Write("Please enter text here: ");
builder.InsertTextInput("TextInput1", TextFormFieldType.Regular, "", "Placeholder text", 0);

// Apply write protection to every section in the document.
doc.Protect(ProtectionType.AllowOnlyFormFields);

// Turn off write protection for the first section.
doc.Sections[0].ProtectedForForms = false;

// In this output document, we will be able to edit the first section freely,
// and we will only be able to edit the contents of the form field in the second section.
doc.Save(ArtifactsDir + "Section.Protect.docx");