If added to Act One, she surely sings it to Sheldrake, as part of her delusional idea that he's going to leave his wife for her. It could be at the top of the Chinese restaurant scene, before he checks out and she must sing "Knowing When to Leave." But that might be crowded. I cannot recall a major Sheldrake/Fran crossover before that. In fact, when I later saw the national company (Anthony Teague, Melissa Hart), I think the audience gasped when she appeared as his mistress. The show plays a suspense line re who Sheldrake needs the apartmenht for, and the audience learns it's the girl Baxter has a crush on.
She wouldn't sing it to Chuck. She OUGHT to look in the mirror and sing it to herself. The character is quite the doormat. But if she sings it to Sheldrake, maybe she'll seem less pitiful.
"I'm a comedian, but in my spare time, things bother me." Garry Shandling