To embed a portal page in another website, you can use a skin with no header or footer and embed it in an iframe element.
See Using Multiple Skins in Using Skins for details on how to use this skin alongside your normal skin.
Here is an example in codepen embedding the Event calendar. This uses the Barebones skin.
Here is another example for a custom form:
<iframe src="https://ministryplatform.com/portal/get_form.aspx?template=embedded&remembertemplate=true&id=21104ba5-026c-42e5-9f2d-6cd5b5eb3e08"></iframe>