How do I sync new icloud account to iMessage on my daughter's ipad

In making assumption that you are not sharing an AppleID between users - and that your daughter has here own AppleID that is signed-in to iCloud on her own device(s) - then iMessage should be signed-in with here own AppleID credentials. Your daughters principal iMessage address will be her primary email address (this is usually the username that identifies the AppleID account)

You can add additional email addresses for iMessage from Settings:

Settings > [Your daughters Name / AppleID] > Name, Phone Numbers, Email > [Contactable At] - tap Edit

Be aware that adding an email address that corresponds with someone else's AppleID may have unexpected consequences.