If you don't already own the exact option (not the stock), then your order would be "sell to open". You are opening an option position.
If you own that exact option (same type, strike, and expiry) and want to sell it, then you would "sell to close". You are closing a position that you currently own.
Note that selling covered calls (calls when you already own the underlying stock) can be fairly risky. You are essentially reducing your exposure to the stock price by giving up any upside profit above the strike in exchange for getting premium upfront. If the stock goes above the strike, then your profit is limited to the strike price plus the premium you receive (minus what you paid for the stock, which is zero for an RSU). If the stock tanks, you get to keep the premium, but bear the brunt of the loss on the stock.