I want to know how can i write a motivational letter to our Hr Manager at my current company?

Hi

We have a vacancy at our company (Head creditors clerk) and i would like to apply. Can you please help me with a motivational letter to my HR? Im currently the creditors clerk at our company. But the problem is there is another girl working with me as a creditors clerk and she also applied for the position and her motivational letter is quite inpressive. I really would like the position and need help!

Thank you