don't say anything to your boss, not yet

Internal transfer is a very tricky process. generally you need to make the hiring department aware that you are interested and have an exploratory or preliminary interview with the hiring manager. It is a mutual agreement that at this stage, you don't need to let your boss know and they will not inform your department, and HR does not know because both parties are only at the initial interest level, far from an actual offer. If the hiring manager thinks that you are a good fit, he will arrange a formal interview. that is the time you need to inform your current line management. HR will let them know anyway.