CustomFieldDataEN[] custs = new CustomFieldDataEN[1];
                custs[0] = new CustomFieldDataEN();
                custs[0].UserID = 1;
                custs[0].ProjectID = issue.ProjectID;
                custs[0].CustomFieldID = YOUR CUSTOM FIELD ID;
                custs[0].CustomFieldData="XYZ";
                issue.IssueCustomFieldData = custs;

Don't work, Sory I'm not sure using this issue.IssueCustomFieldData on _.CreateIssue(issue) or _.UpdateIssue(issue) with custs[0].IssueID=

Which version of Gemini are you running?

Gemini v3.7.2 Build 2784

Can you post the code you are using?

IssueEN issue = new IssueEN();                
                issue.ProjectID = 47;
                issue.IssueType = model.IssueType;
                issue.IssueSummary = model.IssueSummary;           
                issue.IssueLongDesc = model.IssueLongDesc;                 

                CustomFieldDataEN[] customField = new CustomFieldDataEN[1];
                customField[0] = new CustomFieldDataEN();
                customField[0].UserID =_proxy.GetUser(User.Identity.Name).UserID;
                customField[0].ProjectID = issue.ProjectID;
                customField[0].CustomFieldID = 14;
                customField[0].CustomFieldData = model.Customer;               
                issue.IssueCustomFieldData = customField;

issue.IssueID = _proxy.CreateIssue(issue);

Does the issue gets created (issue.IssueID contians a value)? or do you get an error?

Also, set the state of the custom field to New.

Thank, That's work.

customField[0].State = BaseEN.EntityState.New;