Seems very simple, but i cant. Please help me out here.
Can't be done natively in InDesign. You have to do it in Acrobat (which is clearly a pain if there will be edits and you need to go back and forth between InDesign and Acrobat).
If it's a short form, you can use the free edition of FormMagic: http://www.id-extras.com/products/formmagic
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