[1.12.2] How to add recipes to Metal Former via api?

The metal former recipes are accessed via three fields in ic2.api.recipe.Recipes: metalformerExtruding, metalformerCutting and metalformerRolling. The method you'll want to be using is

boolean addRecipe(IRecipeInput input, Collection<ItemStack> output, NBTTagCompound metadata, boolean replace);

The input type you can create using the IRecipeInputFactory instance inputFactory in Recipes, the output is just a list with a single ItemStack for what you want the recipe to produce, metadata can be null for a metal forming recipe, and replace is optional for if you want to override a pre-existing recipe. For a simple example, adding a rolling recipe turning an apple into stone, you'd do:

import java.util.Collections;
import net.minecraft.init.Blocks;
import net.minecraft.init.Items;
import net.minecraft.item.ItemStack;
import ic2.api.recipe.IRecipeInput;
import ic2.api.recipe.Recipes;

IRecipeInput input = Recipes.inputFactory.forStack(new ItemStack(Items.APPLE));
ItemStack output = new ItemStack(Blocks.STONE);
Recipes.metalformerRolling.addRecipe(input, Collections.singletonList(output), null, false);

For the different input options like ore dictionary support, fluid containers, multiple items per recipe, or various different items to produce the same output, take a look at ic2.api.recipe.IRecipeInputFactory.