Would anyone be able to help me in making a new subclass? I need to combine two existing classes/subclassess in order to show progression in an additional class due to my game going beyond level 20 now.

Sure, I can help. What seems to be the problem?

I can't seem to figure out if I'm able to create a subclass in the homebrew options that can/will incorporate both the subclass from my 1-20 levels and my new subclass, which will be my 20+ levels. Example: I am a level 19 Warlock of The Undying with 4 levels of Fighter Samurai. I know I can't go over level 20 in the normal way, but I was hoping to be able to at least add my fighter levels via a custom subclass.

Sure, just copy the class features from the fighter class and subclass as features for your warlock subclass. You’ll just do the first 4 levels for now, but when you level up as a fighter you’ll add more features.

I get lost adding the fighter class features to the copy of the undying. Creating the class feature; such as the bonus proficiency options from level 3 of samurai

Bonus Proficiency

When you choose this archetype at 3rd level, you gain proficiency in one of the following skills of your choice: History, Insight, Performance, or Persuasion. Alternatively, you learn one language of your choice.

and adding it to the undying sublass. I don't understand how to make it so that I'm given the option to choose from the list in character creation.

I also feel like I'm doing a terrible job explaining where my problem is coming in...

I guess I could make a custom of samurai and see how they input the options, no? And just try to copy them into the copy of undying?

Edit: Maybe it's just not possible to basically create a gesalt class/subclass for DnDBeyond.

Yeah, making a copy of the Samurai to see how it works and copying the features onto the warlock subclass is precisely the best way to do it. If you need help with the base Fighter features (like Extra Attacks, etc.) then I can help you with those. The only ones that you can’t actually program are the feat options on the extra ASIs, but you can add feats directly to a character sheet so even that’s not a big deal.