Use the saute function of the Instant Pot, when the oil is hot add onions, saute until almost translucent, then add the garlic. Add chicken breast and salsa. Cook on high on the manual setting for 20 minutes. Perform a quick release on the Instant Pot. Remove chicken and shred.
Some people love to shred chicken in the stand mixers, by using the mixer blades on low speed.
Slow cooker cooking times
If you want to use a slow cooker you will need to saute the onions and garlic in a large skillet, then transfer the sauteed garlic and onions to the slow cooker. Add the chicken and salsa to the slow cooker. Cook on low for 4 to 6 hours.