CORS problem with POST https request

I need to send a request to a bank payment API from an Ionic app, using the Angular HTTP Client service. The request shouldn’t triggera CORS preflight because:

1. Use a POST verb
2. The only header added is Content-Type: text/plain
3. The expected response is text/plain.

If I try the request using Postman or Isomnia, I see there is no OPTIONS preflight and the request returns the expected response.

Screen Shot 2021-01-06 at 12.32.181405×842 252 KB

But when I try the same request from Ionic, using this code:

      const headers = new HttpHeaders({
        "Content-Type": "text/plain",
        "Accept": "text/plain"
      });

      const url = "https://..../Authorize3DS";
      this.http
          .post(url, xmlData, {
          	headers: headers
          })

then I get this error, both from local server or from the actual app:

{"headers":{"normalizedNames":{},"lazyUpdate":null,"headers":{}},"status":0,"statusText":"Unknown Error","url":"https://.../Authorize3DS","ok":false,"name":"HttpErrorResponse","message":"Http failure response for https://.../Authorize3DS: 0 Unknown Error","error":{"isTrusted":true}}

I am positive that this is a CORS issue because if I install a CORS-enabler on my browser and retry the request, then it works perfectly. Also, I tried other REST servers under HTTPS and they all work perfectly, so it must be something related to the CORS implementations on the bank’s webervice, but I can’t figure out what could it be and what to do in my ionic app to get this working.

Any ideas?

TIA

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