I have been through this with all my kids. I find perseverance is the key, eventually she will learn she has to stay in the stroller. I remember physically manipulating my child into the stroller, it's not fun.

If she is able to undo her straps then consider getting reigns or other straps and tie them so she cannot reach the buckle to undo them. Also the first few times you get her in, take her for a walk somewhere quiet because the hardest thing is when your child is screaming and trying to climb out and you feel 100 pairs of eyes on you. So go somewhere like a quiet park and persevere until she understands that she has to go in the stroller. As Didspunk says, if you need her to be in the stroller then this is an argument you must not let her win.