How do I send data from .NET web application to billboard or led display using internet

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pardon me if this is too vague for now

It is too vague for a little text box like this - this is "Quick answers" and you don't give anywhere near enough info to even begin answering it in any detail.

So start with the display unit itself: it needs to either be connected directly to the internet (with an IP address or programmable so it can "announce" it's availability to your IP address) or be connected to a device which is.

Then look at what it's manufacturer says about it: how does it suggest you display data?

Only then can you even start to work out how to actually program code to work with it!