Passing uint256 param in writeContract in Typescript

[jlee88my]

I have an ABI that has a uint256 parameter input (tokenQuantity).
In Typescript, writeContract translate that uint256 to bigint.
However, the content of tokenQuantity is larger than bigint (eg 10,000,000 * (10^18) --I use Decimal to calculate that).
How do I pass this as arg in writeContract? I tried tokenQuantity.toFixed() to convert it to string first. However, writeContract complains that it needs a bigint instead of a string.

There must be something simple I am missing as this is very basic to pass uint256 (quantity) as part of a parameter.
Please help. I am stuck :'(