Hi Baowen,
What version are you running? Can you try updating to 7.3.9? This snippet works for me just fine when I click sign in a PDF document:
instance.setSelectedTab('signatureModal', 'textSignaturePanelButton');
annotManager.setIsAdminUser(true)
annotManager.setCurrentUser('unique user');
Is this the behaviour you were expecting?:
Cheers,
Dustin