D&D 5E Multiclass question

[doctorbadwolf]

If it's something very specific, such as the Bladesinger, I would expect it to be part of the character's backstory. If it isn't part of the character's backstory, I wouldn't allow the subclass to be selected unless they had found a bladesinger to teach them.

Is the multiclass allowed to be part of their background? Ie, they’ve been training their whole life to be a Knight of Volustan, which involves mixing the precision, mobility, and panache of the Swashbuckler with the stalwart dedication to god and country of the Crown Paladin.

Or does background only work for subclasses?