Header Button Cutom Font

Site URL: https://www.players-social.com/

Heya, does anyone know how to use custom font for header button. I manged to change the buttons on the site but not the button in the header: https://www.players-social.com/ Thanks very much

Use this CSS for header button

header#header a.btn {
    font-family: 'SportingGrotesque-Regular' !important;
}