Login in your Account Portal, Click on Phone Numbers. Select the number you want to configure to set up call forwarding.
Click on any number and select "Edit Configuration" to go to configuration screen.
Select SIP -> SIP Account and select your SIP Account from the list.
Click on Save.
This is how the Phone Number will appear after applying the configuration. Check the image below.
There are lot or advanced options, Click on the "?" to see the description for each option.