To specify such an injection you should provide a profile file as follows:
((injectionflowrate transient 12 1)
(time 0.0000 0.0001 0.0030 0.0031 0.2500 0.2501 0.253 0.2531 0.5 0.5001 0.503 0.5031 0.75 0.7501 0.753 0.7531 1.0 )
(flowrate 0.0 0.001 0.001 0.0 0.0 0.001 0.001 0.0 0.0 0.001 0.001 0.0 0.0 0.001 0.001 0.0 0.0 ) )
This profile would be read from File > Read > Profile and be selected for the Flow Rate of the injection.
The Start Time would be set to 0 s and the Stop Time to some big value, say 1000 s.
Note that a time of 1e-4 s is assumed to change the value from 0 to the required flow rate. This is done for the demonstration only and this time interval should be adjusted as per the need of your specific case considering time step size, flow rate values etc.