To custom a field on Jira:

Step 1: Log in to JIRA Server as an administrator

Step 2: Choose Settings button at top right corner

Step 3: At drop down menu, select Issue

Step 4: At the left hand side, choose Custom fields option

Step 5: Click on button

Step 6: Select an approximate field type then custom it

Step 7: Create an issue on Jira to view the result