Play animation only when a key is being held down

First you will have to create a parameter, it can be of the bool type:

Then create a State, with the name Transition, in Create State> Empty

Then place your animation and create two transitions, round trip, look:

On the outward transition, deselect Has Exit Time and click on the "+" to add the parameter, leave it as true.

In the transition back, do the same thing but leave the parameter false.

Then replace your script with this one

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class StrobeButton : MonoBehaviour
 {
 
  public Animator knop;
 
  // Start is called before the first frame update
  void Start()
  {
 
  }
 
  // Update is called once per frame
  void Update()
  {
      if(Input.GetKeyDown(KeyCode.Space))
     {
         knop.SetBool("Bool", true);
     }
     if(Input.GetKey(KeyCode.Space))
     {
         knop.SetBool("Bool", true);
     }
     if(Input.GetKeyUp(KeyCode.Space))
     {
         knop.SetBool("Bool", false);
     }
  }
 }

You can use tracker bools to keep track of whether space is still being held and whether the animation is currently playing. For example:

     private bool AnimationIsPlaying;
     private bool SpaceIsHeld;
 
     void PlayAnimationOnKeyHeld()
     {
         if (Input.GetKey(KeyCode.Space))
         {
             SpaceIsHeld = true;
         }
         else if (Input.GetKeyUp(KeyCode.Space))
         {
             SpaceIsHeld = false;
         }
 
         if (SpaceIsHeld)
         {
             // if animation isn't already playing
             if (!AnimationIsPlaying)
             {
                 knop.Play("StrobeDown");
                 AnimationIsPlaying = true;
             }
         }
         else
         {
             // Stop playing animation
         }
     }

Running the method in the Update should ensure that the animation only is played while space is held, and when let go, the animation should stop playing. @vershurengiovanni