Angular - How to do http.Post with query parameters

I am working with the Strava API and I have the following question. If I try from Postman with my client_secret and with the corresponding code it works for me

But this, taken to Angular, I'm going crazy to make it work, right now I have this, and it gives me all the time bad Request, without more information.

Let's see if you can give me a hand to launch the request, because, I think, I'm putting those params wrong (the ones you see in Postman)

    const headers = new HttpHeaders()
    const body = JSON.stringify({})
    const params = new HttpParams()
      .set('client_id', environment.client_id)
      .set('client_secret', environment.client_secret)
      .set('code', code)
      .set('grant_type', environment.grant_type)

    this.http
         .post<any>('https://www.strava.com/oauth/token', body, {
           headers: headers,
           params: params
         })
        .pipe(catchError(this.formatErrors))
        .subscribe((res) => console.log(res))

EDITED

I have added the .append, as you mention and I still get the same error.

    const headers = new HttpHeaders()
    const body = JSON.stringify({})
    const params = new HttpParams()
      .append('client_id', environment.client_id)
      .append('client_secret', environment.client_secret)
      .append('code', code)
      .append('grant_type', environment.grant_type)
      
    this.http
         .post<any>('https://www.strava.com/oauth/token', body, {
           headers: headers,
           params: params
         })
        // .pipe(catchError(this.formatErrors))
        .subscribe((res) => console.log(res))

And the error it gives is the following:

EDIT 2

I have continued working, I have created an injector to control the errors, but I am going crazy, I do not understand why it works from Postman and not from Angular

I enclose the capture of the errors and how the headers and the body are formatted before launching the request. It annoys me that they are not of type Key - Value and that they are Key - Array. I don't know if this is normal

On the other hand I attach the new code to see if you see something strange

refreshToken(code: string){
    
    const headers = new HttpHeaders({
      'Content-Type': 'application/x-www-form-urlencoded'
    })

    const body = new HttpParams({
      fromObject : {
        client_id: environment.client_id,
        client_secret: environment.client_secret,
        code: code,
        grant_type: environment.grant_type
      }
    })

    this.http
        .post<any>('https://www.strava.com/oauth/token', body, {
          headers: headers
          //params: params
        })
        .subscribe((res) => console.log(res))
  }

In case it served to clarify something, I leave here the direct link to the documentation , but I see everything correct