Can I embed a code to build the form I want?

If I understand correctly, you want to embed a custom code to your form so you can customize it to your liking? If you are referring to adding codes such as javascript or php, then it is not possible to do so on the form builder. You can only add these codes if you use the full source code of the form and host it on your own server.

If you are referring to adding custom CSS code only to design your form, then that is possible to do in the form builder. Simply follow this guide: How to Inject Custom CSS Codes. Regards.