Yes, you need to create the form with Acrobat using Acrobat's Form Tool. Look under the menu option "Tools =>Forms" and select "Show Forms Toolbar" or the form field type you want to create. Acrobat is not really designed for creating forms but adding form fields and form interactivity, but you might not be able to use this feature with the PDF/A-1b standard. You should create the base form in another application that is designed for authoring documents and then add the form fields. You will need to make sure your PDF confirms to the PDF 1.4, version 5, standards.
Since PDF signatures require an external resource they might not be covered, you should check the ISO standard.
Thank you for answer. I have tried to create form containing one TextBox in Acrobat Prof 9, but when I try to save as PDF/A-1b, I get error "XMP property neither predefinden nor defined in extension schema".
And yes, Acrobat is realy very weak form-designer.
Do you know any application, that can produce PDF/A-1b forms?