This sounds like one of the Realm of the Elderling books by Robin Hobb. Specifically this would be Fool's Errand, the first book of the third trilogy, The Tawny Man.
Prince Dutiful has gone missing just before his crucial diplomatic wedding to an Outislander princess. Fitz’s assignment to fetch Dutiful back in time for the ceremony.
In the chapter 'Charms' just over 1/3 of the way through a Hedge Witch called Jinna makes a charm for Fitz in the form of a necklace that is supposed to make people like him.
'This is a variation on a very old love charm. I have made it not to attract lovers, but to make people well disposed towards you, if it works as I hope it will. When you try to create a variation on a theme it often lacks strength. Sit still now.'
She walked behind my chair with the dangling necklace...
While Fitz is wearing the charm at this initial setting, the witch who made it starts blushing while looking at him, while her much younger niece invites Fitz to stay for dinner. Jinna cautions him to keep it covered, but find an excuse to loosen his collar if he is in a situation where it may help.