Something convincing.

You rack your brain to think of something convincing to say, and after a few seconds, you come up with something. You try convincing Grillby that you would be able to get along very well with the customers, persuading them to come back to the pub again, thus making Grillby more money. As such, you should be getting paid more than what was originally offered. Despite not having a mouth, you swear you can see Grillby smirking slightly. You and Grillby stare at each other for a moment, until you give in under the stare and sign the application without further complaint, handing both it, and the pen, back to Grillby. After giving the job application a quick once over, Grillby nods his head and offers to shake your hand. Despite your hesitance at shaking hands with someone who appears to be made of fire, you do it anyway. Grillby's hand feels just a bit warmer than what you expect a human's hand to feel like. It's also more solid than you were expecting, which confirms that those muscles you saw earlier aren't just for show.

After finishing your handshake, Grillby takes his rag and goes back to wiping down the counter. You don't think you're being dismissed, though it's hard to tell. You suppose you could tell Grillby you would like to start working right away, but you could also talk to one of the other patrons here, or even Grillby, in order to pass the time until it gets busier. You could even leave the pub and come back to work another time if you wanted to.