Sir/Madam,
Respectfully, I am __________ (name) and I am working as ___________ (designation) in your reputed company for last _________ (duration). My staff ID number is __________ (mention your staff ID).
I write this email with the utmost respect to inform you that I am willing to take casual leave for __________ (number of days) from __/__/____ (date) till __/__/____ (date). Most happily, I would like to inform you that my cousin is getting married on __/__/____ (date).
Therefore, it is to request you kindly approve my leave request at the earliest and I shall be highly obliged for your kind support. In case of need, I will be available on calls in this regard, you may contact me at _________ (mention your contact number).
Yours faithfully,
_________ (Name),
_________ (Employee ID Number),
_________ (Contact details)