I would translate it as same as you did if YOU want to lose YOUR weight, or "I wish/hope I can lose my weight". However, there can be other translations depends on the context, for example, someone wish/hope that the other one can lose the weight. I hope this helps you.

It can be a correct translation, if you are making a wish or praying. It also can mean wishing something to someone. For example: Friend "To lose some weight I''ve started yoga, but I'm not sure it would work" and you answer "やせますように!(I believe in you and wish you succeed)". Quite a weird conversation though.

For my perspective, やせますように means: I will lose my weight. It is a decision or target,but not a wish.