The person who can walk away from the deal controls the deal.

Whoever needs it least sets the terms. That is not a rule anyone wrote down, it just falls out of the situation, because the side that cannot leave has to accept whatever keeps the thing alive. Being able to walk is not a threat you make. It is a fact the other person can sense in how you sit.

Build the alternative before you need it: another option, a bit saved, a second person who would take you. You may never use any of it. But you will negotiate completely differently knowing it is there, and that difference shows up in the result.