Issue : integration of redmine Issue with Gitlab on prem

Hi ,
I have integrated on-premise redmine with on premise gitlab vesion 12. i can see an link of redmine in gitlab page.while clicking link , i am able to redirect to redmine page.
my problem is , while commiting the code in gitlab that time i wanted to mentioned the issue id of redmine using [#Issue_Number ]. this is as per documentation. but i am unable to link issue id as its not appearing. i am trying # then Ctrl+TAB to see redmine issue id .but no luck.
How to solve this issue . please help