When you get an ITA, this is what is mentioned for experience letter:
You must provide proof of work experience for your current job and for each past position you listed. Proof must include a reference letter from your employer and pay stubs, if you have them. The reference letter must:
-be an official document printed on company letterhead
-include your name, the company’s contact information (address, telephone number and e-mail address), the signature of your immediate
-show all positions held while employed at the company
-include these details: job title, duties/responsibilities, job status (if current job), the dates you worked for the company, the number of work hours per week and your annual salary plus benefits.
You must scan all documents for this period of employment and save them as one file. (You must create a separate file for each period of employment.)
Now this reference letter cannot be addressed to a third party like USCIS. It needs to be addressed either to you or to no one in particular (to whomever it may concern). If your employer is not willing to give you a letter, take a printout of the email where they are denying this letter. Ask a co-worker/manager from that company to provide a letter mentioning your title, responsibilities, dates that you worked there, work hours, salary (if possible) and their work relationship with you (manager, colleague, subordinate etc.). If you have the paystubs, attach some of them if possible.