RxJS how do I make POST requests in sequence where a new request is fired only when the previous completed?

Here is a simplified scenario:

   of([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])
  .pipe(
    map((data) => {
      let arr: Observable<any>[] = [];

      for (let elem of data) {
        let obj = {
          title: 'foo' + 'elem',
          body: 'bar' + 'elem',
          userId: elem,
        };

        arr.push(
          this.http.post(
            `https://jsonplaceholder.typicode.com/posts`,
            obj,
            {
              headers: {
                'Content-type': 'application/json; charset=UTF-8',
              },
            }
          )
        );
      }

      return arr;
    }),
    concatMap((data) => forkJoin([...data])),
  )
  .subscribe((data) => console.log(data));

My understanding is that in this case all requests are going to be fired in parallel because that's what forkJoin([...data]) does. I'm trying to figure out how I can fire a request only if the previous one completed (succeeded or failed, doesn't matter).

I'd appreciate any suggestions or help. Thanks.