A Form is to all intents and purposes a document container (ODT or ODS) with a layout of form objects. In order to set a margin on a form, you need to be in Form Design/Conception mode (which allows you to set the page boundaries). The same would apply to centering the table grid object you show in your post, you would do this in Form Design mode, then save the form, switch back to Data Entry mode and print it out from there. You might find you get better results when the form is exported to PDF before attempting to print (or not, as sometimes form object positions aren’t maintained correctly on PDF export).