Forms SSO prefill is compatible when using the API Connector on a Workflow, but there are a number of things to consider while combining these features. If you need more information on our API Connector and setting it up for the first time, please check out this article. If you're looking for more information on setting up SSO prefill with your Form, please check out this article.
SSO prefill will prefill forms in a workflow so long as the user has configured SSO prefill for that specific form.
In those cases, if a user also sets up an API Connector to prefill steps on that form, the SSO prefill logic is going to win out. In other words, there is no point in a user trying to "override" a field prefilled with SSO prefill with an API connector step and this will be the same behaviour irrespective of field type.
A common example or use case of how these features can be used in tandem is a user incorporating SSO prefill for the very first form in a workflow, so end users can authenticate and view prefilled values via SSO. After that first form step, you may want to incorporate an API connector to allow for new information to display on additional steps in a workflow.