Before starting off coding your G Code files, please feel free to visit this link below for further information: 



Kindly add this sequence into your G Code as follows: 

M750 T1 ; opens valve for extruder 1 
G4 P500 ; sets amount of delay, "P" being milliseconds (see G-Code guide)  
M751 T1 ; closes valve for extruder 1  

Please note that T0 stands for the first printhead, T1 for the second printhead and so on. 

For further assistance regarding this matter, kindly send your queries at