You don't need to officially 'accept' the expression of interest. If you decide to go with OINP application, just go to Ontario's One Key portal and register your account. Under this One Key account you can create a new OINP application (You can create and withdraw OINP applications as many times as you want), and for each OINP application you have 14 days to upload all documents. Among those documents, they will ask you to upload this NOI message (letter), which you can convert into a pdf or image, as a proof. This NOI is valid for 6 months, meaning that from the date you receive this message, you have 6 months to complete the above application submission before it expires. Hope it helps!