Make a field required when a value is selected inside a select list field.

const changedField = getChangeField();

const isRequired = changedField.getType() == "com.atlassian.jira.plugin.system.customfieldtypes:select"
    && changedField.getName() == "Ticket Priority"
    && changedField.getValue().value == "Must have";

getFieldById("customfield_10038").setRequired(isRequired);