Idea of Add to Favorites Icon
There is a "plus" sign which is like a cross, but the sides are even, centered inside a five pointed star with all sides that are equal. Both are outlined in black. The star is "hollow" otherwise" in that there is empty space surrounding the icon. Everything is straight (there are no curved lines).
This page shows the different variations of the Add to Favorites from an Icons8 icon pack. Feel free to resize and recolor the icon, and then download the icon in PNG format for free.