The RTMP encoder should be configured to send multiple streams at different bitrate and these streams should be sent to the Millicast stream/destination. Once the multi-bitrate RTMP streams are received by Millicast service they can be simulcast to the viewers.
The RTMP streams being sent from the origin should have the same Millicast server path and key but each with a different source-id. This article about RTMP MBR to WebRTC Simulcast (ignore the OBS part) explains how the stream path and key need to be specified. Specifically, read the section 'Here is example using 3 bitrates (1080p, 720p, 360p)' where the stream path and keys are shown.