In the slow cooker insert, whisk together the chicken broth, orange juice, marmalade, soy sauce, sesame oil, rice vinegar, chili sauce, grated ginger, minced garlic, and 2 tablespoons of the cornstarch until well blended. (Set the other tablespoon aside for later.)
Transfer the chicken breasts into the slow cooker and spoon some of the sauce over them to cover.
Secure the slow cooker lid, then cook on low for 3-4 hours or on high for 1.5-2.5 hours. (This may vary by slow cooker) Tip: If you can, keep an eye on the internal temperature of the chicken as it cooks with a meat thermometer. You want the chicken to reach an internal temperature of 160-165ºF.
Once the chicken breasts are cooked, transfer them to a cutting board and cut them into bite-sized cubes.
If the slow cooker is cooking on low, switch it to high. Mix 1 tablespoon of cornstarch with 1/4 cup of cold water, then whisk it into the slow cooker sauce. This will thicken it up a little more.
Once the sauce has thickened, change the cook setting to “Keep Warm”. Transfer the cubed chicken back to the slow cooker, add the orange zest, and stir to coat the chicken. You can add the scallions at this point or save them for a crisp garnish.