If you haven't already, be sure to follow the Setting Up Planning Help Doc and add your Church ID to the Planning Center Add On on your dashboard.
Once that is done, you'll be able to use the Planning Center Form Modal.
Embedding A Planning Center Form
Planning Center doesn't allow their forms to be embedded but they do allow them to be launched in a Popup. Here's how to open a Planning Center Form in a popup.
Add a new button to any block or page header.
Change the Button Type to URL
Enter the link to the Planning Center Form into the URL field.
At the end of the URL field add ?open-in-church-center-modal=true to the url.
Whenever someone clicks that button the PCO modal will launch and show the form.