Is there a template for my motivation letter?
No, there is no official template. Your motivation letter is a personal statement in which you explain why you wish to study at Leiden University and why you have chosen this particular study programme. It is an opportunity for you to convince the faculty of your qualities and goals. The letter should contain about 700-1500 words.
Note: if you are applying for a programme at Leiden University College (LUC) you should follow the guidelines issued by LUC on the writing of motivation letters.