Swagger/Swashbuckle: OAuth2 with Resource Owner Password Credentials Grant

OK, I solved it like this:

Add a JavaScript completion-handler to swagger:

config
    .EnableSwagger(c => {
                    //do stuff
    })
    .EnableSwaggerUi(c => {
        c.InjectJavaScript(typeof(Startup).Assembly, "MyNamespace.SwaggerExtensions.onComplete.js");
    });

Take username:password from the API_KEY textbox:

$('#input_apiKey').change(function () {
    var key = $('#input_apiKey')[0].value;
    var credentials = key.split(':'); //username:password expected
    $.ajax({
        url: "myURL",
        type: "post",
        contenttype: 'x-www-form-urlencoded',
        data: "grant_type=password&username=" + credentials[0] + "&password=" + credentials[1],
        success: function (response) {
            var bearerToken = 'Bearer ' + response.access_token;
            window.authorizations.add('key', new ApiKeyAuthorization('Authorization', bearerToken, 'header'));
        },
        error: function (xhr, ajaxoptions, thrownerror) {
            alert("Login failed!");
        }
    });
});

Thank you @Dunken. Your answer almost solve my problem, but to make it work with latest Swashbuckle version I had to change it a bit like this

$('#explore').off();

$('#explore').click(function () {
   var key = $('#input_apiKey')[0].value;
   var credentials = key.split(':'); //username:password expected

$.ajax({
    url: "yourAuthEndpoint",
    type: "post",
    contenttype: 'x-www-form-urlencoded',
    data: "grant_type=password&username=" + credentials[0] + "&password=" + credentials[1],
    success: function (response) {
        var bearerToken = 'Bearer ' + response.access_token;

        window.swaggerUi.api.clientAuthorizations.add('Authorization', new SwaggerClient.ApiKeyAuthorization('Authorization', bearerToken, 'header'));
        window.swaggerUi.api.clientAuthorizations.remove("api_key");
        alert("Login successfull");
       },
       error: function (xhr, ajaxoptions, thrownerror) {
        alert("Login failed!");
       }
    });
});