How to call plugin method within new issue page?

I am new to redmine and creating sms sending plugin and trying to integrate it with issue module (within New Issue).

I have created one button and one custom field (text field) on "new issue" page/form?
Can you guid me how can i access/get the custom field value and where should I put the button click event handler code? Also I would like to know how can I call Plugin method when button is clicked?

Any tutorial are also welcome.