I would wait for the offer. By the time they've made the offer, they have already invested in hiring you and they just want to pay you as little as they can get away with to do so. That is your moment of maximum leverage - and when you can choose whether to say you have other offers as suggested below.
In general, figure out what new hires for the job is paying locally and increase your ask by at least 10% of that and/or 10% over their offer, whichever is higher. You might already know what you want to get paid and shouldn't go below that either, of course.
Keep in mind that very few companies give proportional raises for time worked vs. what they offer new hires. This is your best chance of getting more pay at this company.
I would wait for the offer. By the time they've made the offer, they have already invested in hiring you and they just want to pay you as little as they can get away with to do so. That is your moment of maximum leverage - and when you can choose whether to say you have other offers as suggested below.
In general, figure out what new hires for the job is paying locally and increase your ask by at least 10% of that and/or 10% over their offer, whichever is higher. You might already know what you want to get paid and shouldn't go below that either, of course.
Keep in mind that very few companies give proportional raises for time worked vs. what they offer new hires. This is your best chance of getting more pay at this company.