Integration with iFirma needs a few steps and information about your accounting.
At first, you must have iFirma token which you will find in your profile after you log in to iFirma.
On the top bar, you can find your ifirma login to use it in the integration form in the Zencal.
Go to the API tab
Click the green button and generate Api Key. Then you can copy this key and paste it into the Zencal integration form.
The last step is to set your accounting information to let the Zencal create invoices properly.