how inject 1mW power to antenna

Connect it to the network analyser (VNA) and set the output power of 0 dBm which is equivalent to 1mW. Please see reference below.

dbm_to_watt_conversion

I think he is trying to say "how do I get 1 mW of power into a system with an antenna attached to the front end".

Well, you need to get another antenna, put much more than 1 mW into it, point it at your receive antenna, and somehow calibrate the mess so that 1 mW is received into the antenna's effective aperture.

u mean, i could just use a RF signal generator to set the power of 1mW n connect it to the antenna?

If your question is what I think it is then yes, it is as simple as that.

However, biff44 did get me rethink a little. If biff44 is right, then his proposed method should be considered.