I would only use the in such a case if you were already talking about that microwave, or the need for it is obvious from context, or if you were describing where everything is in your kitchen. Otherwise, you're not really talking about a specific microwave, just the one that happens to be yours.
"Can I heat up this ready meal?"
"Sure, the microwave is in the kitchen."
(Note that it could be "there's a microwave in the kitchen" and be equally natural and idiomatic.)
"Please describe your kitchen."
"Well, the cooker is over here, and the fridge is over here, and the microwave over here."
(Again, "there's a cooker over here" and so on is also acceptable.)
"I heard you got some new kitchen equipment."
"Yeah, the microwave is set up in the kitchen."
"Shall we watch the film?"
"Sure, just let me put this popcorn in the microwave."
(There's only one microwave you could be putting it in - the fact this is about something you're going to do makes a different.)
But when talking about it and what matters is that it is a microwave, you could use the indefinite article.
"Is there some way to heat up water?"
"Sure, I've got a microwave."
(However, you could also respond in a way that talks about a specific action and it becomes the instead: "Sure, I can do that in the microwave.")
"What appliances do you have?"
"Well, there's a cooker, a dishwasher, a fridge, a microwave..."
Sometimes, knowing when to use the definite article versus the indefinite article isn't a matter of applying straightforward rules. You just kind of have to get to know it by feel. Practice, practice, practice, as they say.