Most people negotiate with one offer. That is a normal position, not a weak one you have to decorate. This page sets the script to say, in plain language, that you do not have another offer and that you are not going to pretend you do. The ask rests on the target, the scope, and the market note you type.
What you can point to
Use a range you looked up for the level and the city, or the band the company posted. Put the source in the market box in your own words. Use the work they already said they want: on-call, hiring, a migration, a book of clients. Those are facts from the interviews. They are stronger than a mysterious “other opportunity.”
You can still ask for a signing bonus, a clearer grant, or a level check. None of those require a second employer. If the gap is large, ask whether the level is right before you repeat the number. A low level with a lecture about confidence will not move a band that was built for a different job.
The sample still includes a target above the priced total, so you can see a real counter. If your offer is already at your target, the script will say so instead of manufacturing a fight.
Questions
Is it harder to negotiate with only one offer?
You have less calendar pressure. You do not have less of a case if the number is off for the level. Say the target and the reason. Stop.