Tips/ideas on how to get my 15 month old to drink fluids oth

I have a daughter that just turned 15 months on the 30th. She still nurses which I love. I do not have a problem at all with still breastfeeding her, but we are trying to get her to start drinking milk or pediasure drinks because of her weight. She has always struggled to gain weight since she was born. She had heart problems in the beginning that were causing her to struggle to gain, but now they don't feel like that is the issue. She is healty and she will eat solid foods but she wants nothing to do with milk or really any liquid other than breastmilk. I have tried every bottle and sippy cuppy I can find and she just is not interested in it. She is 15 months and only weighs 15 lbs. They have checked to make sure she isn't anemic and all kinds of things. Everything came back good she is completely healthy she just doesn't gain weight very fast. They want me to get her to drink milk and stuff to try to get extra calories in but she just won't do it. Anyone have any ideas or tips for a breastfed baby that won't drink anything else?

I don't know if this helps much, but my dd rarely takes more than a few sips of anything if I give it to her (other than breastmilk). But if I leave a sippy of water by her toys she takes drinks of it all day.

With milk or pediasure you'd have to change it out frenquently but maybe she'll take it if she discovers it. Dd always acts like she found a treasure.