I opened a ticket with AWS and they still do not support this in 2025. They sent over this comment describing a workaround, "The only workaround available is to configure Google client as an OIDC identity provider in the Cognito user pool [2] and have the initial redirect from the Hosted UI hit an interceptor instead of Google that adds that query parameter before redirecting to Google with the HTTP request. Means, while configuring your Google client as OIDC provider [2] you need to provide the authorization endpoint URL as the URL of an interceptor so that Cognito forwards the request to your interceptor. That interceptor needs to add the 'prompt=select_account' query parameter and need to redirect the end user to Google's authorization endpoint with 'prompt=select_account'."