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Before you can edit a Workitem in a JSF Form you need to create an new empty instance of a Workitem. Therefore you can call the ActionListener Method "doCreateWorkitem" form you WorkflowController. The method creates an empty worktiem which is assinged to a specific processID provided by the model inside your Workflow instance. You can bind the ActionMethod to a commandButton or commandLink from a menue:

The following code example shows a command button which creates a new wokitem assigned to the ProcessID "100" form the Model version "0.0.1".
<h:commandButton
value="create new...."
actionListener="#{workflowMB.doCreateWorkitem}"
action="show_workitem">
<f:param name="id"
value="0.0.1|100" />
</h:commandButton>If you don't deal with model versions you can also simply provide a valid ProcessID without a model version
<h:commandButton
value="create new...."
actionListener="#{workflowMB.doCreateWorkitem}"
action="show_workitem">
<f:param name="id"
value="100" />
</h:commandButton>