I multiple roles on my website, I want to assign a certain role to certain users when they register to my site. How can I assign a specific role to new users programmetically when they submit their registration forms. I have tried modules like logintoboggan, autoassignrole but they provide default user role on registration but I want to assign role to specific users and not all users signing up to my site.

I have multiple roles on my website, and I want to assign a certain role to some users when they register to my site. 

How can I assign a specific role to new users programmatically when they submit their registration forms?
I have tried modules like logintoboggan, autoassignrole, but they provide a default user role on registration, when I want to assign role to specific users, not to all users signing up on my site.