How to add/give NBT tags to items through a procedure?

What are you trying to do with it? Like custom data?
This worked in a custom code snippet for setting the unbreakable component,

itemstack.applyComponents(DataComponentMap.builder().set(DataComponents.UNBREAKABLE, new Unbreakable(true)).build());

Well you can use the "new" consumable component to apply effects when you consume it, but it gets quite long

itemStack.applyComponents(DataComponentMap.builder().set(DataComponents.CONSUMABLE,
		new Consumable(
				2f, // consume time
				ItemUseAnimation.DRINK,
				SoundEvents.GENERIC_EAT, 
				true, // has consume particles
				java.util.Collections.singletonList(new ApplyStatusEffectsConsumeEffect(new MobEffectInstance(MobEffects.OOZING), 10f))
		)
).build());

sure,
implementing it is easy, just put it in a custom code snippet like https://mcreator.net/comment/284160/#comment-284160
 

# Breakdown
so you can basically just ignore the first line itemStack.applyComponents(DataComponentMap.builder().set(DataComponents.CONSUMABLE,
It basically is just like telling the code that the following lines will be a consumable component which will be added to the item.

Then we create the actual consumable component, each parameter explained in comments (lines starting with //)

new Consumable(
		// The amount of time it takes to consume
		//   the item might be in ticks
		2f,
		
		// The animation to play when the item is
		//   being consumed, there are quite a few,
		//   the most common ones being;
		//   ItemUseAnimation.DRINK, and ItemUseAnimation.EAT
		ItemUseAnimation.DRINK, 
		
		// The sound that should be played when the item is consumed
		//   you can find a list of built-in sounds here,
		//   https://mappings.dev/1.21.4/net/minecraft/sounds/SoundEvents.html
		SoundEvents.GENERIC_EAT, 
		
		// If particles should be shown
		//   when the item is consumed
		true,
		
		// And finally the thing you're problably interested in,
		//   this defines what potion effect to apply when the
		//   item is consumed.
		//
		//   The first part of it,
		//   `java.util.Collections.singletonList` is just telling
		//   java to make the following into a list.
		//
		//   Then, `MobEffects.OOZING` sets the potion effect type
		//   to apply to the oozing effect,
		//   And, 10f sets the duration of that potion effect to
		//   10, probably ticks.
		java.util.Collections.singletonList(new ApplyStatusEffectsConsumeEffect(new MobEffectInstance(MobEffects.OOZING), 10f))
)

Hope that helps, feel free to ask if not!