Retrofit continue work in browser (response as html page)

I'm working with a bank api. When user chooses to add his bank card to his account I need to make request to their API and then continue in browser for user to enter his card credentials in https safety

API looks like this:

@FormUrlEncoded
@POST("/jsp/hbpay/logon.jsp")
Observable<Response<String>> addCard(@Field("Signed_Order_B64") String order,
                                   @Field("email") String email,
                                   @Field("Language") String lang,
                                   @Field("BackLink") String backLink,
                                   @Field("PostLink ") String postLink,
                                   @Field("FailureBackLink") String failureBackLink);

BackLink and others are supposed to be web links to send user in different situations. I don't know what to put there for android.

Next, I have my request:

private void addCardRequest() {
    showProgress();
    mDisposable.add(createAddCardOrder()
            .flatMap(orderString -> mBankApi.addCard(orderString,
                    mDataManager.getLocalUserInfo().getEmail(),
                    BankApi.EPAY_LANG,
                    BankApi.EPAY_BACK_LINK,//"http://localhost/"?
                    BankApi.EPAY_POST_LINK,
                    BankApi.EPAY_FAILURE_BACK_LINK))
            .subscribeOn(Schedulers.io())
            .observeOn(AndroidSchedulers.mainThread())
            .subscribe(this::handleAddCardResponse,
                    this::handleError,
                    () -> Timber.w("addCard completed")));
}

Response is a plain html page with a bunch of pics and scripts. What I'm doing for now is just saving it as a .html file and then displaying in a webview:

private void handleAddCardResponse(Response<String> response) throws Exception {
    hideProgress();
    if (response.isSuccessful()) {
        String fileUrl = getContext().getCacheDir() + "/add_card.html";
        Utils.writeStringToFile(getContext(), response.body(), fileUrl);
        startActivity(WebViewActivity.getIntent(getContext(), fileUrl));
    } else {
        throw handleUnsuccessfulResponse(response);
    }
}

What I need is to open incoming response (which is a html markup) in browser, or to make the browser make my request and handle response