For my custom button with Content Source = URL I use following formula text to get instance domain:
LEFT($Api.Partner_Server_URL_390, FIND('/services', $Api.Partner_Server_URL_390))
that returns "https://vfirst-dev-ed.my.salesforce.com/"
But when I click the button it navigated me to following URL that is includes necessary part with encoded domain:
Please assist, what I do wrong? How I can just go to domain URL when I click the button?