You can add a form to your page by clicking on Forms library on the left sidebar:
From there, you can choose to select an already-created form, use one of our templates or create a new form.
To learn more about creating forms, please refer to the dedicated article here: https://d.pr/3yDW36
After adding a form to the page, you can change the form and button style and choose whether the form will be visible on both Desktop & Mobile or just one of them.
In order to test the form submission, make sure you publish the page first, as it cannot be tested in Preview Mode.