I would suggest looking at the documentation.
A quick search found me this to access the form fields:
// For complete examples and data files, please go to https://github.com/aspose-pdf/Aspose.PDF-for-.NET
// The path to the documents directory.
string dataDir = RunExamples.GetDataDir_AsposePdf_Forms();
// Open document
Document pdfDocument = new Document(dataDir + "GetValuesFromAllFields.pdf");
// Get values from all fields
foreach (Field formField in pdfDocument.Form)
{
Console.WriteLine("Field Name : {0} ", formField.PartialName);
Console.WriteLine("Value : {0} ", formField.Value);
}
Source where I found it: https://docs.aspose.com/pdf/net/extract-form/
this is from the example on how to populate form fields:
// For complete examples and data files, please go to https://github.com/aspose-pdf/Aspose.PDF-for-.NET
// The path to the documents directory.
string dataDir = RunExamples.GetDataDir_AsposePdf_Forms();
// Open document
Document pdfDocument = new Document(dataDir + "FillFormField.pdf");
// Get a field
TextBoxField textBoxField = pdfDocument.Form["textbox1"] as TextBoxField;
// Modify field value
textBoxField.Value = "Value to be filled in the field";
dataDir = dataDir + "FillFormField_out.pdf";
// Save updated document
pdfDocument.Save(dataDir);
Source: https://docs.aspose.com/pdf/net/fill-form/