Baldur's Gate 3 allows you to romance all of your companions, but some of them are more easy-going than others. In the case of Shadowheart, there's an item that you can only get in Act 2 of the game while you're in the Shadowlands called the Night Orchid which will progress your romantic relationship with her.
Where to find the Night Orchid in the Shadowlands
There's a decent chance that you'll come across the Night Orchid naturally in your travels across the Shadowlands but in case you don't, you need to find the Shadowed Battlefield fast travel spot in the map. It's just east of the Last Light Inn, and you'll find the Night Orchid at the following coordinates: X: 39 - Y:37. Just pick it up and add it to your inventory, then you can use it the next time that you're at the camp.
When you return to the camp, talk to Shadowheart and choose the dialogue option that mentions the Night Orchid, telling her that you remember it being her favorite flower. She'll thank you and take it off your hands, progressing your relationship with her and advancing the Daughter of Darkness storyline, allowing you to get to know Shadowheart on a deeper more personal level.