Choosing the right opamp for biasing a square wave
Technically, you don't need the opamp at all; your capacitor, feeding it to a higher-than-source impedance resistor to your reference voltage does exactly that.
So, if the original source of your square wave can drive your load that you plan to attach to Vout, this simple CR high pass filter totally biases the square wave to whatever you want.
The capacitor value would hence be chosen to be large enough to put your cutoff frequency below 1 MHz. That depends on the resistor.