Not just anyone can burp on command, because some people are also unable to burp due to a condition called no burp syndrome also called Retrograde Cricopharyngeus Dysfunction (R-CPD).
Retrograde Cricopharyngeus Dysfunction (R-CPD) is a medical condition in which someone cannot burp or they find it very difficult burp.
Retrograde Cricopharyngeus Dysfunction (R-CPD) is caused by a tight tight cricopharyngeus muscle in their throat, which traps gas in their stomach and can cause discomfort.
People with Cricopharyngeus Dysfunction (R-CPD) may also have bloating, abdominal fullness and chest discomfort and it may sometimes be treated with Botox injections to relax the muscle.
People that burp on demand or burp on command often learn to burp on command by swallowing of air, and practicing techniques like swallowing a gulp or a carbonated drink or by using their jaw movements to create suction, which forces the gas out.
To try burping on command, try gulping air without swallowing food.
You can try practicing by pushing that trapped air out from your throat.
And drink a carbonated beverage like soda, which is a common way of triggering a burp.
And even some gentle exercise or stretching can also help move the trapped gas and relieve pressure.
With practice you can also often learn to use your jaw movements and proper technique to burp in a more controlled way.