This is probably the most commonly asked question of all time.
It only takes a couple of lines of code for you to set the value into a workflow variable. The coding is as below.
import org.joget.workflow.model.service.*
WorkflowManager wm = (WorkflowManager) pluginManager.getBean("workflowManager");
wm.activityVariable(workflowAssignment.getActivityId(),"status", "Approved");