It was a very simple letter he wrote. He just stated that I am a current employee at his company and that part of my duties included meeting with clients to discuss business related things (like pricing, logistics etc). He then put that he would like for me to meet with one of our clients in Mexico. He also put that I am one of the more qualified and available employees to actually go and meet the client.
It was fairly short, like a 5 sentence paragraph. Simply said i worked for him and that he would like me to go meet a client. Did not submit any "proof" or emails from the client or itinerary or anything like that. I did include a letter myself though, stating that I was DACA and that i needed to travel for work to meet a client so i was applying under the Employment category. The main reason i submitted my own letter was to request for it to be expedited. Unfortunately they did not expedite it, but it did get approved in about 6-7 weeks.