Get a subject line written with the draft, or write it yourself with a field.
Open a sequence from "Sequences".
Click "Add step".
Choose "Email".
Leave the subject empty.
Click "Draft with AI".
Put subject constraints in the prompt: length, tone, no questions.
Click "Generate Draft". The subject is written with the body.
Close the panel.
Or keep control: a short subject with a merge field often beats a generated one.