I want to run 20A constant current and 4.2V on a load using a variable DC power supply rated same
I'd be charging that battery in a purpose built charging bag, and wearing Nomex overalls (you, not the battery) while doing so would probably do no harm. 

You also want to be sure that the supply will genuinely deliver 20A at 4.2V. 

Assuming that   


the battery CAN tolerate the rate you say, 
The supply is capable of safely delivering 4.2V at 20A for 3+ minutes.
The supply current limit can not be set when not loaded.


Disconnect any load.
Set supply to say 4.25V.
Set CC control to minimum.
Short supply output terminals
Adjust CC control until I = 20A
Unshort terminals.  

The supply is now ready. When loaded below 4.2V it will deliver 20A.
When Vload rises to 4.2V it will deliver 20A or less. 

Connect supply to fully discharged battery with correct polarity.
After 3 minutes the battery WILL NOT be fully charged.
This is because when Vbattery rises to 4.2V the supply will go into CV mode ND current will be set by the battery. It will start to taper down.
CV to CC change over will probably happen in the 2 m to 2m 30s range.
This is a natural consequence of LiIon / LiPo chemistry.

IF you are sure that the batter will tolerate (for some values of tolerate) a constant 20A input then the charger should be set to 20A CC as above AND the supply voltage set "higher.
MAYBE 4.5V. Maybe 4.7 Maybe - flaming ruin .... .
This is not how the books ever say you can do it.
this violates the most fundamental principles of LiIon charging.
This sounds like fun :-)

If it works, more power to you, but battery life will be shortened so it will be less energy to you overall long term.