How is the L2 regularization derived? [duplicate]

I just proved to myself why the regularization is added rather than multiplied to loss function.

I did so by taking the MLE formula...

$$\operatorname{argmax}\sum \log(P(x_i\mid\Theta ))$$

and since we know that MAP uses a prior belief distribution...

$$P(\Theta \mid x) = \frac{P(x\mid\Theta )P(\Theta )}{P(x)}$$

We can write MAP as...

$$\operatorname{argmax}\sum log(P(x_i\mid\Theta )P(\Theta)$$

If we redistribute the logs, we can see that $$\log(P(\Theta))$$ is the regularization terms, as shown below...

$$\log(P(x_i\mid\Theta)) + \log(P(\Theta))$$

but now I would like to show how L2 itself is derived. L2 is defined as...

$$\lambda \sum_k \sum_l W^2_{k,l}$$

which is the element-wise multiplication of the weights. Where did this equation come from? What values for $$P(x_i\mid\Theta)$$ and $$P(\Theta)$$, for example, do I need to use to derive this L2 formula? Can someone please explain it to me step-by-step?