Being in NIW, you don't have to have it but I prefer you to include it.
To Whom It May Concern:
This is to confirm that Mr. Ph.D is a [your title] in my lab, with a annual salary of $00k starting from mm/dd/yyyy. Any question please contact me.
Your boss
To Whom It May Concern:
This is to confirm that Mr. Ph.D is a [your title] in my lab, with a annual salary of $00k starting from mm/dd/yyyy. Any question please contact me.
Your boss