Is there a deadline you have to meet for submitting this form?
If yes, I would do it as 11ant suggested, write a letter to your general contractor with a deadline, asking them to send you the completed, stamped, and signed form (to be safe, set the deadline you give the general contractor to at least one week before the official deadline expires) – with the "just for completeness" note that if you do not receive the form and therefore lose the funding of X euros (feel free to insert the exact amount), you will recover the money from the general contractor. And if this is even regulated in your general contractor contract, you can also refer to that again.
Send the letter by registered mail with proof of delivery and you’re done. And if you’re paranoid, you can also find a witness for putting the letter in the envelope and dropping it in the mailbox...