<post-functions>
   <function type="class">
      <arg name="class.name">com.bnpparibas.jira.function.issue.MyCustomFieldFunction</arg>
   </function>
</post-functions>

public class MyCustomFieldFunction implements FunctionProvider {
	...
	public void execute(Map transientVars, Map args, PropertySet ps) throws WorkflowException {
		GenericValue issue = (GenericValue) transientVars.get("issue");
		MyCustomFieldHelper.createMyCustomField(issue);
	}
	...
}

public class MyCustomFieldHelper {
	...
	public static ChangeItemBean createMyCustomField(final GenericValue issue) {
		ChangeItemBean cib = null;		
		// Retrieve MY_FIELD Custom Field
		CustomField cf = cfMgr.getCustomFieldObjectByName(ModelConstants.CF_MY_FIELD);
		if (cf.getValue(issue)==null) {
			cib = cf.updateValue(issue, "A New Value");
		}
		return cib;
	}
	...
}

public class MyCustomFieldFunction implements FunctionProvider {
	...
	public void execute(Map transientVars, Map args, PropertySet ps) throws WorkflowException {
		Issue issue = (Issue) transientVars.get("issue");
		MyCustomFieldHelper.createMyCustomField(issue);
	}
	...
}

public class MyCustomFieldHelper {
	...
	public static ChangeItemBean createMyCustomField(final Issue issue) {
		IssueChangeHolder changeHolder = new DefaultIssueChangeHolder();
		// Retrieve MY_FIELD Custom Field
		CustomField cf = cfMgr.getCustomFieldObjectByName(ModelConstants.CF_MY_FIELD);
	
		if (cf.getValue(_issue)==null) {
			ModifiedValue modifiedValue = new ModifiedValue(null,"A New Value");
			if (_issue instanceof MutableIssue) {
				((MutableIssue)_issue).setCustomFieldValue(cf,modifiedValue.getNewValue());
			} else {
				cf.updateValue(null,_issue, modifiedValue,changeHolder);
			}
		}
		return changeHolder.getChangeItems();
	}
	...
}

<workflow-function key="generatechangehistory-function" name="Generate Change History" class="com.atlassian.jira.plugin.workflow.WorkflowNoInputPluginFactory">
     <description>Updates change history for an issue and stores the issue to the database.</description>

     <function-class>com.atlassian.jira.workflow.function.issue.GenerateChangeHistoryFunction</function-class>

     <orderable>false</orderable>
     <unique>true</unique>
     <deletable>false</deletable>
     <weight>300</weight>
     <default>true</default>
     <addable>non-initial</addable>

     <resource type="velocity" name="view" location="templates/jira/workflow/com/atlassian/jira/plugin/generatechangehistory-function-view.vm"/>
</workflow-function>