You can do so in the mission, A Clash of Forces, Chapter 9 of Fire Emblem Engage. Use Diamant’s Talk Ability to start a conversation with her. After the dialogue sequence, she will be convinced ...

This encounter will occur just after you meet with two princes, Diamant and Alcryst ... You can romance Ivy in Fire Emblem Engage. Sage is also an adequate choice for Ivy, leaning more toward ...