How to use PSM and eDRX mode in custom code?


We are using nRF9160 and want to use PSM & eDRX mode in one of our project.
We referred 'Asset tracker Application' & found that lte_lc_psm_req() and lte_lc_edrx_req() are APIs to activate/deactivate corresponding modes.
Please correct me if I am wrong here.

Using above mentioned 2 APIs require changes in prj.conf & Kconfig.
But, as we are customisig our code from the scratch, we dont have Kconfig file.

Can you please tell us how to prepare these files & how to add them in a project ?

Thank You,