How do I add a phone number to iMessage for iPad

You have to have an iPhone number to add for that to work. See this.

Add or remove your phone number in Messages or FaceTime - Apple Support

I asked the hosts to remove your screenshot. It is showing your Apple ID email address and this is a worldwide forum. You don’t want to publicize your Apple ID to the world.

On your iPhone, go to Settings>Messages>Text Message Forwarding and select your iPad. Follow the prompts to set it up.

See also:

How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac - Apple Support

I could very well be wrong, heaven knows it won’t be the first time, but the OP made this statement which I took to mean that they don’t have an iPhone.

i dont have a phone signed in with my apple id, what can i do to add a phone number on my ipad

Demo wrote:

I could very well be wrong, heaven knows it won’t be the first time, but the OP made this statement which I took to mean that they don’t have an iPhone.

i dont have a phone signed in with my apple id, what can i do to add a phone number on my ipad

And, as I've done before on more than one occasion I misread the original post! Thanks.

Then you cannot add a phone number to iMessage on your iPad. Having an iPhone is a requirement.