Let $ABCD$ be a convex quadrilateral such that $AB + BC = 2021$ and $AD = CD$. We are also given that $\angle ABC = \angle  CDA = 90^o$. Determine the length of the diagonal $BD$.
1. Given that $ABCD$ is a convex quadrilateral with $AB + BC = 2021$ and $AD = CD$. We are also given that $\angle ABC = \angle CDA = 90^\circ$.
2. Since $\angle ABC = 90^\circ$ and $\angle CDA = 90^\circ$, the sum of these two angles is $180^\circ$. Therefore, quadrilateral $ABCD$ is cyclic (a quadrilateral is cyclic if and only if the sum of a pair of opposite angles is $180^\circ$).
3. By Ptolemy's Theorem for cyclic quadrilaterals, we have:
   \[
   BD \cdot AC = AB \cdot CD + AD \cdot BC
   \]
4. Since $AD = CD$ and $\angle ADC = 90^\circ$, triangle $\Delta ADC$ is an isosceles right triangle. Therefore, the length of the hypotenuse $AC$ is:
   \[
   AC = AD \sqrt{2}
   \]
5. Substituting $AD$ for $CD$ and $AC$ in Ptolemy's Theorem, we get:
   \[
   BD \cdot (AD \sqrt{2}) = AD \cdot (AB + BC)
   \]
6. Simplifying the equation, we have:
   \[
   BD \cdot \sqrt{2} AD = AD \cdot 2021
   \]
7. Dividing both sides by $AD$, we get:
   \[
   BD \cdot \sqrt{2} = 2021
   \]
8. Solving for $BD$, we have:
   \[
   BD = \frac{2021}{\sqrt{2}} = \frac{2021 \sqrt{2}}{2}
   \]

The final answer is $\boxed{\frac{2021 \sqrt{2}}{2}}$