Material Design required field

Has anyone seen in Material Design how to present a required field? At the moment it seems the guideline is either to hide the submit button until required fields are populated or else throw an error on submit.

I have seen an example where "required" was written in brackets beside the field, and also an example where "not required" was written, but this is a bit underwhelming.

Does one just go with the good old red asterisk in the label? (In this case, it is prompt text and would be turned into the label when the field is populated.)