How to apply RGB to Pantone colour in photoshop application

Open file in photoshop 

image/mode/rgb color

File save

if that is what you are trying to achieve.

Your post isn't clear.

Do you wish to create a 2 colour image, black and spot red from an RGB image? Something like this:

Should the black knock out or overprint on the red?

What version of Photoshop are you using?

What is the intended print method?