code: public EpicIssue assignIssue(Object groupIdOrPath, Integer epicIid, Integer issueIid) throws GitLabApiException {
          Response response = post(Response.Status.CREATED, (Form)null,
                  "groups", getGroupIdOrPath(groupIdOrPath), "epics", epicIid, "issues", issueIid);
          return (response.readEntity(EpicIssue.class));
      }
docs: Creates an epic - issue association. If the issue in question belongs to another epic
  it is unassigned from that epic.
  
  <pre><code>GitLab Endpoint: POST /groups/:id/epics/:epic_iid/issues/:issue_id</code></pre>
  
  @param groupIdOrPath the group ID, path of the group, or a Group instance holding the group ID or path
  @param epicIid the Epic IID to assign the issue to
  @param issueIid the issue IID of the issue to assign to the epic
  @return an EpicIssue instance containing info on the newly assigned epic issue
  @throws GitLabApiException if any exception occurs