You may use the following draft for your e-mail:
Subject: Permission to Work in HR Department
As you are aware, I am working in _______ (name of your company) since ________ (date). I had applied for the post of ________ (name of the post) and my interview was also conducted for the same post. Nevertheless, on my selection, Mr ________ (name of the partner) sent me mail asking me to handle dual role of Frond Desk Officer (FDO) and also as HR Executive. Though I started working as FDO since _______ (date), I am yet to get chance to work in HR Department.
Sir, while placing me at the job of FDO, I was told that I will be shifted to HR Department after three months. Now five months are over since I started working as FDO. Therefore, I hereby make earnest plea to restore my placement at the HR Department.
I have completed _____ (your qualification in HR) and thereafter have acquired experience of three years as HR Executive. I wish to pursue my career in the field of HR. However, there is a mismatch in my career ambitions and the work that is allocated to me. Compatibility of present work with that of past will keep me motivated. Prolonged interlude with non-HR activities is not conducive for my career. Hence this request.
________ (your complete name)
22nd January 2018 From India, Bangalore