If you have a Behance profile associated with the same Adobe ID, the integration will automatically be turned on.
When starting out with Adobe Portfolio, your Behance pages will import to your default Gallery. When you click into Pages in the left-hand panel, you will see your Behance pages (marked with a Be symbol) within your Default Gallery.
You can change the default import Gallery from the Integrations option in the left-hand toolbar. Click into box and and select your new Default import Gallery:
Note: this does not move your existing imported pages to the new Gallery, only newly created Behance projects will appear in the Gallery.