Getting error message 403 on post request

I´m building a custom CMS to our discourse platform.
I’m having some trouble when trying to post something.
I’m building it in angular and the programing language is typescript.
Here is a picture of my function

 createACompetition(CompetitionName: string, groupName) {
    const headers = new HttpHeaders().set('Content-Type', 'multipart/form-data');
    const data = new FormData();
    data.append('api_key', WLIApiKey);
    data.append('api_username', WLIUsername);
    data.append('color', '49d9e9');
    data.append('text_color', 'f0fcfd');
    data.append('name', CompetitionName);
    // Security group on create
    // data.append('permissions[]' + groupName, '1');

    return + '/categories', data, {

and this is my error message

Did I make a mistake in my post request ?

My guess is that the API key is wrong or the user didn’t have permission.

No, that not the problem.
I Think it has something to do with headers?