When to use it
Use it when you have a competing offer and genuinely prefer this role. Mentioning the other offer gives you leverage, and saying this one is your first choice keeps it friendly. Only say it if it's true.
Example
You: “I have another offer, but this role is my first choice. Is there flexibility?”
Hiring manager: “Let me see what we can do.”
In an email or Slack message
By email: “I've received another offer, but this role is my first choice. Is there any flexibility on the base or start date?” Be ready to share the other offer's details if asked.