Hello I just got a beard transplant or facial hair transplant how do I use the salt water spray solution?

Every doctor has different post-operative care instructions.  Some don't use the spray at all.  Call your doctor.

I have never prescribed a salt solution transplant for any part of the body as it leaves residual salt on the skin. My washing routine is as follows and it applies to any hair transplant anywhere in the body. An hair transplant procedure is treated just like a regular hair
transplant with regard to the recipient area, but if the donor area has open
wounds with an FUE, we require daily washing with soap and water. Within 3 days of
surgery, you can resume full activities, heavy exercises if you wish. The
recipient area requires daily washes as well to keep the recipient area free of
crusts. I generally recommend the use of a sponge and supply my patient with a
surgical sponge to fill with soapy water and press on the recipient area daily.
By repeating this daily, all crusts can be washed off without any fear of
losing grafts. IF any crust are present, use a Q tip and dip it into soapy
water, and roll it on the crusts and that will lift them off without dislodging
them, but never rub them, just roll the Q tip on the recipient crust. I like to
see no evidence of any crusting in the recipient area and the crusts from the
donor area gone in 7-10 days with daily washing

your hair loss specialist should have given you instructions. Hieever, at our clinic we have the client spray every one to two hours on the recipient area for the first two days, let it soak and don't towel dry..  After that they can start using the special shampoo that we provide for cleaning the first 7 days.