As with the other questions (1) (2) this can be accomplished using the 2019-01-01-preview version of the API that is not in the documentation. As with the others, as a preview feature, the endpoint could change without notice.
I've discussed this with the product team, and they are working on an API update that will be a later API version than 2019-01-01. Once those updates are ready, the documentation will be updated with that newer version.
URL:
POST https://management.azure.com/subscriptions/{TenantID}/resourcegroups/{ResourceGroup}/providers/microsoft.labservicesl/labaccounts/{LabAccountName}/labs/{LabName}/sendEmail?api-version=2019-01-01-preview
Body for this one is fairly simple:
{
"emailAddresses": ["{email 1}", "{email 2}"],
"extraMessage": "{custom message}"
}