Circuit element: Rectangle with diagonal
That is the symbol for a relay coil. The relay should be a DC type with a coil voltage equal to the DC power supply.

24 V DC is common in industrial control systems and, since your switch is rated for 300 mA max., the minimum coil resistance can be calculated from \$ R = \frac {V}{I} = \frac {24}{0.300} = 80 \ \Omega \$.

The switch is NPN type which means that there is an NPN transistor internally that will connect the black and blue wires internally when the switch is 'on'. This may also be called a 'switched negative'. The relay '+' terminal is always connected to the positive supply.