This tutorial is for ring around the collar but that is just another type of sweat stain and this should work well https://www.hometalk.com/3682130/how-to-easily-remove-a-shirt-collar-stain?r=2
White vinegar let the stained part just sit in the vinagar lightly brush with tooth brush then rinse with cold water soak one hour
I hope you have good luck doing this! Stains are so difficult at times.
Mix 1/4 cup water with 4 Tbsp baking soda. Scrub in and let sit on stain. Wash in cold water. Repeat (before drying) if necessary.
Soak in lemon juice for about an hour. Place in the sun until dry. My mom used to do this to our white tennis shoes every month!
Do not wash the lemon juice out! Wash normally after sun bath.
Thanks , can’ t wait to try some