During Act 3, explore the Disused Passage accessible via Harbormaster's Office and western Warehouse Districts areas of the Docks (Night) and find a Rivaini Talisman in a box labelled "Storage".
Go to the Hanged Man and speak to Isabela. Hawke will give the necklace to Isabela, who explains that the necklace is a Rivaini fertility necklace that is meant to increase fertility and bring about love.
Note: Even if you have romanced Isabela fully and she claims she has fallen for you, she still acts hesitant about love, saying that she'd rather stick with lust.
If Hawke is male and you flirt with Isabela you go upstairs and have sex.