Adding more info:
From the scenario description, it might be an option to create a custom edit and new-form for the list that needs to include data from the WCF service.
See for example Creating PowerApps SharePoint ListForms for Custom Lists
In this form, you would access the WCF service to populate a dropdown field with data from the SQL database, but would store the selected value in a normal text field of the list.
This info contains a URL to a third-party article, I'm just sharing here for your reference. The URL is not controlled by Microsoft.