calling function inside payment module using ajax request

[shaho]

I need to call a function inside my payment plugin (in /vmpayment/ folder) using an ajax request, the problem is that I don't know what URL to call
Should probably be something like "siteurl.com/index.php?option=vmpsplugin...etcetc". I am making the call from the select payment page in the checkout. Appreciating any help on this.