Being able to sit still at dinner time and eat is a learned skill. It takes patience on the parent's part while the child learns this. At first she will be wiggly and unfocused but if you allow her to be this way while encouraging her to sit still and eat she will eventually learn.
I recommend that you stop feeding her. Sit her in a chair that is high enough for her to comfortably be seated and still reach the table. Place the food in front of her. Give her a spoon and encourage her to take a bite with you guiding her hand if you haven't gone thru the process so far.
Accept that she will be wiggly and allow a certain amount of it. When she loses focus give her the spoon and guide her in eating.
Allow for 30 minutes to do this. As she learns the time will shorten. Be patient and consistent.
This is the process that most parent's and kids go thru starting at 2 or so. Perhaps you found it easier to feed her then to wait for her to learn.
I also agree that if she doesn't want to eat, then she doesn't have to but she doesn't get anything to eat until the next meal or snack time. She will not starve. She will eat when she's hungry. And she will feed herself if you don't feed her.
Do you give her lots of finger foods that she can pick up for herself? That may also help. Do not expect good manners until she's learned how to feed herself.