d = {'key': 'value'}
print(d)
# {'key': 'value'}
d['mynewkey'] = 'mynewvalue'
print(d)
# {'key': 'value', 'mynewkey': 'mynewvalue'}

you create a new key\value pair on a dictionary by assigning a value to that key. If the key doesn't exist, it's added and points to that value. If it exists, the current value it points to is overwritten. —R. Navega