If you're using MSI afterburner you should be able to unlock the core voltage control in settings > general tab > compatibility section > "unlock voltage control [third party]". Though this won't do anything for you, you won't see any performance difference. This feature is for hard-core overclockers who have flashed their graphics card with a modified VBIOS (GPU firmware) that allows for the card to pull more voltage than the manufacturer intended. For example, my 2080 Ti VBIOS allows the GPU to pull 1.093V out of the box. If I unlocked the voltage control and put it at 100% on MSI afterburner, the card will only pull 1.093V because that's what the VBIOS allows, but even without doing this, my GPU already pulls 1.093V - so my performance remains the same, nothing's changed. Now, hypothetically if I flashed my 2080 Ti with a modified VBIOS that would draw 1.1V, then this feature would actually affect my performance but kill my card much faster.
So conclusion, unlocking voltage control won't do anything if you're running on a stock VBIOS and I wouldn't recommend flashing your VBIOS to a modified version that allows more voltage since it'll kill your card quicker. Though if you realllllllly want to try, you'll need to scower the web for a modified VBIOS for YOUR SPECIFIC MODEL CARD that allows more Voltage to be pulled. Though I do not recommend this for novices as you may brick/destroy your card permanently in the process if you do it wrong.