You can link your Spotify account when you first sign up for a Sentric account. If you haven't done that yet, you can do this anytime.
Select the option 'songs' from the main menu, on the left side.
You will see the song suggestion section at the top of the page. Click on the 'link your Spotify' button.
Search for your Spotify account. Select the correct account from the list, then click on the 'link account' button.
It should take 30 minutes at the most for your Spotify songs to filter through and appear in the song suggestions to add directly to your account.