Hi @Robert1925,
Welcome to the EE Community
You can pay for third party subscriptions via your EE mobile bill as long as Spotify premium has the option for this payment method you should be able to set it up.
If it does not let you then you may have a bar on third party charges active on your mobile number. If you have only one number on the account you can remove this by texting UNBAR to 150, if you have more than one number you would need to call us to remove the bar.
Alex
