The stagnation inlet boundary will have conditions set that correspond to a Mach number of 0.75. The equivalent free stream velocity is roughly 300 m/s so this is the value you will use to initialize the velocity field.
Open the Initial Conditions node in the Physics 1 node. Open the Velocity node and select the Constant node.
In the Properties window, set the Value property to 300,0,0.
STAR-CCM+ adds the decimal point and units automatically, so by simply entering 300,0,0 you get an entry of [300.0, 0.0, 0.0] m/s.
In the same Initial Conditions node, open the Turbulent Viscosity Ratio node and select the Constant node.
Set the Value property to 50, which is the same as the turbulent viscosity ratio that will be set on the stagnation boundary condition.
Save the simulation.