How do i calculate square magnitude

[sharp parrot]

So i have the following dilemma:

It’s nothing complex, you just want a force that pushes against the direction an object is moving, that scales with the square of its speed.****

How do i calculate sqrt magnitude?
linvel is linear velocity in Vector3 format {x,y,z}
dot is dot product
sqrt is squared root?

Here's an example of code i got so far, but it's wrong:

            let drag = -0.1; // coefficient?
            let drag_force = drag
                * velocity.linvel.normalize() //normalized 
                * ((velocity.linvel.dot(velocity.linvel)).simd_sqrt()); // sqared magnitude?

[eager sedge]

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