After some research, found that we can create the Epic Link custom field using below code.
MutableIssue epicIssue = ComponentAccessor.getIssueManager().getIssueObject("Epic_Key");
jiraIssue.setCustomFieldValue(customFieldObject, epicIssue);
The program runs good. However it doesn't show up my Epic Link on Issue Screen.
When I ask "Where is my field" it says, the Epic Link value is not set, therefore it is not visible.
Any idea on this? How can I make my Epic Link visible when I create the Epic Link programatically?
Thanks in advance!