As reported by Censor.NET, this is stated on Poroshenko's Facebook page.
The statement reads: "We have had a three-hour discussion with representatives of the November 21 Initiative in order to allay their concerns about the constitutional reform. It seems that the meeting's participants have heard enough to grasp the need for change, as well as the risks of reform's failure."
"From the standpoint of domestic policy, it is a key reform needed for all others to succeed. Speaking of external benefits, it is a matter of retaining Ukraine's international support, effective sanctions against the Russian Federation and guarantee of our victory in the war of independence and territorial integrity."
"Not the least argument: if we see Moscow criticizing our efforts, then we are on the right track," the statement resumed.