The statement above is false.
If the diagonals of a parallelogram form right angles, then the parallelogram is a rhombus (a rhombus is a quadrilateral with four equal side lengths).
Note* = by saying the statement is false is not saying that the scenario presented in the statement cannot occur. If the rectangle was a square, then its diagonals can form right angles since a square is also a rhombus. However, if a rectangle was NOT a square, its diagonals would not form right angles. A true statement is a statement where ALL cases fit the said requirement(s).
The statement can also be corrected by saying:
If the diagonals of a parallelogram are congruent, then the parallelogram is a rectangle.
All rectangles (even a square) have congruent diagonals, so this statement would be true.
Hope this helps!