Make your ask in a professional way and not as an ultimatum. Thank them for the offer and say that you are very excited about the opportunity.You can negotiate and offer "anytime" before you accept and start. I hope this finds you well. Learn to craft an effective salary negotiation email with tips and templates. Highlight your achievements and research to support your case. Read your offer letter from top to bottom at least three times. Did they send you their benefits package? All you need to say it, "I appreciate you considering it! I'm interested enough in the job that I'd love to accept regardless.