how can i add a form that i've created to Facebook as part of a post? instead of a adding it as a tab.

I am sorry to say that the only way is adding your form as a tab on your Facebook Page. Please, check the guide below for more information:

https://www.jotform.com/help/22-Adding-a-Form-to-Your-Facebook-Page

If you have any question or issues, please do not hesitate contacting us. We will be glad to assist you.