Since Manitoba is a province, not a city, the definite article is mandatory (
le Manitoba, not just
Manitoba). The definite article however contracts with the preposition
de to
du.
Bonjour du Manitoba !
So, if switching from Manitoba to Quebec, the article clarifies whether you are talking about the province or the city:
Bonjour du Québec ! → province
Bonjour de Québec ! → city