using Adaptavist Script Runner plugin > Behaviour
def desc = getFieldById ("description")
def defaultValue = """ *Some text in bold*
since we have a multiline string, we can put some more
but keep it short, this is a template...
What / Why / How ...""".replaceAll(/ . /, '')
if (! underlyingIssue.descrption) {
desc.setFormValue(defaultValue)
}