How to set Secret variable for gitlab-runner locally?

Hi All, I have recently setup gitlab-runner. It is working fine on hosted envvironment but locally it is not working. I have set secret variable on gitlab named SSH_PRIVATE_KEY. How do I set this variable locally as I am getting it as blank.
Version: 10.7.2
Git revision: b5e03c94
Git branch:
GO version: go1.8.7
Built: 2018-05-14T11:52:36+00:00
OS/Arch: linux/amd64