Once you create a Service Account, you need to manually harvest the user tokens using a utility such as the API Test Tool (https://apitesttool.desire2learnvalence.com/) to authenticate with your LMS. You then need to store those keys securely, and configure your LMS to ensure the user tokens are long-lived. Many systems have a token timeout of 30 days, but when a headless integration is in place like the one you're proposing, it's often a good idea to make the timeout infinite. You can contact Desire2Learn Support to verify the timeout value for the user tokens.
There's a similar question that addresses this issue as well: Authenticaton Method for Desire2Learn REST API vs SOAP.