How to Gain Forward Speed and Momentum with a Rigidbody?

 public float diveRate = 1.0f;
 public float minDiveSpeed = 10.0f;
 public float maxDiveSpeed = 15.0f;

 float divePos = 0.0f;
 
 void FixedUpdate()
 {
      
      if(Input.IsButtonDown("Dive Button"))
      {
           divePos = Mathf.Clamp01(divePos + diveRate*Time.fixedDeltaTime);
      }
      else
      {
           divePos = Mathf.Clamp01(divePos - diveRate*Time.fixedDeltaTime);
      }
      
      rigidbody.velocity = new Vector3(rigidbody.velocity.x, -Mathf.Lerp(minDiveSpeed, maxDiveSpeed, divePos), rigidbody.velocity.z);
  }

That should work :D

[Edit] If you want to change the rate of diving, simply alter "dive rate" and it will cause you to go into a dive faster or slower, respectively. As well, changing the "minDiveSpeed" and "maxDiveSpeed" will alter how fast you dive.

I used velocity because it would give you a more exact, and easily-calculated speed. However, if you want other things to alter the speeds, as well, you may want to use AddForce, and just calculate how much force is going to be applied, and how long it will last before it will need to be applied, again.