How to type argmin over a variable in plain TeX

I am forced to use plain TeX instead of LaTeX and I don't know what is the proper way to type argmin with some variable under it.

I know

$$\min_b.$$


And I would like to do someething similar, but with argmin instead of min.

$$\argmin$$


doesn't exist :(

Please, does anyone have the knowladge how to do this?

Define it just like \min is defined:
\def\argmin{\mathop{\rm argmin}}