if the brake lever is firm, then the brake should not need more brake fluid.
There is a small torx screw on the lever - position the lever so this is uppermost to preven air getting into it and remove the scew but be careful not to lose or damage the tiny oil ring. Then add a drop of two of fluid by, for example dipping a screwdriver in brake fluid and then holding the end over the hole so the fluid drops off.
Then refit the screw. if you have a syringe, then you can fit that to the lever and draw the plunger to draw air out of the master cylinder.
If the lever is spongy, this means you have air in the system so the topping up of fluid in the brake lever is not going to firm up the brakes - you would need to bleed them using the two syringe menthod. if ytouy need to do that, be patient, in my experience it can take well over 15 mins to get the air out, a mind numbliing tedious process that make you want to hurl yourself off a tall building into a very spikey hedge.