I'm trying to create an item or feat to give an Extra Attack. Tried to use Bonus->Extra Attacks-> Fixed Value 1

Also tried Set->Extra Attacks->Fixed value 1.

I've tried to set the Fixed value to different numbers as well, but nothing seems to work. 

I'm testing feat/item on a Fighter 15 / Monk 5.

The purpose of this is to allow that Extra Attack that Multiclassing doesn't allow. For example, Monk 4 / Fighter 4 only grants 1 Extra Attack instead of 1 for each.

Is there something I'm missing? I save everything, I refresh pages and equipping the item or feat won't add the extra attack. 

Helpful comments only, please

It doesn’t work like that. If Extra Attack comes from more than one source, it will always only ever use the higher one. The only way to increase Extra Attacks is to set the number as 1 higher than the highest one they already have.

Do you mean setting the Fixed value higher? Because I tried setting it to 5 at one point just to see if it would work and that didn't take either.

Did you try it on a feat, a magic item, or a magic weapon?

If you tried it on a magic weapon it will only work for that weapon. So Set->Extra Attacks->1 will grant the wielder 2 attacks with that specific weapon only. If the character already have 2 or more Attacks it should not apply any changes whatsoever.

If you tried it on another type of magic item, it might not work at all. It might also not work with feats, I haven’t tested that. Not every modifier works for every type of homebrew. Alternatively, it might be something that requires Attunement to work when applied to a magic item. Did you try that?

Finally, whenever you add or change anything on a homebrew, you must re-save that entire homebrew using the main save button in order to trigger the system to pick up the changes. After that, the time it takes to push changes to the character sheet may vary. Sometimes it could be instantaneous, at other times it can take up to 15ish minutes. 

I haven't tried it on a weapon, just items and feats. I've done the full sacing process and waited a while. Just isn't working. It might not be possible on a class or classes that already get Extra Attacks

The only way I've found to make this work is using:

Modifier Type: Set
Modifier Subtype: Extra Attacks
Fixed Value: some number

Where "some number" is the number of extra attacks total., including the number of extra attacks a character's class and level already give. If you're adding 1 extra attack, you have to fill in the Fixed Value according to the following table:

Now when you set the Modifier Type to Bonus, Extra Attacks is an option in Modifier Subtype, but setting any Fixed Value does nothing. I'm not entirely sure why Extra Attacks is an available subtype for the Bonus modifier type. The list of subtypes is already different between Bonus and Set, so you would think Bonus/Extra Attacks should work, but I cannot find any evidence that it does.

This limitation makes it difficult to make custom items or feats that affect Extra Attacks since you can't just say +1 to whatever your character already has given. You have to make 4 different options the PC or DM has to choose from depending on the PC's class and level. My example of this is some homebrew I've been working on that adds all of the effects of the Haste spell. As you can see,  I have to have a version for each of the 4 options from the table above:

• https://www.dndbeyond.com/magic-items/1940336-add-effect-haste-30-speed-1-attack
• https://www.dndbeyond.com/magic-items/1940343-add-effect-haste-30-speed-2-attacks
• https://www.dndbeyond.com/magic-items/1940366-add-effect-haste-30-speed-3-attacks
• https://www.dndbeyond.com/magic-items/1940487-add-effect-haste-30-speed-4-attacks

And if you're wondering why each of those say "30-walking-speed" in the link is because I have not found a way for homebrew to "double" speed. You have to use a similar method:

Modifier Type: Bonus
Modifier Subtype: Speed
Fixed Value: some number

Where some number is the already derived speed which can be a combination of: race, heavy armor penalty, Mobile feat, class features, Boots of Speed (which don't customize the character's Speed because it's not an Always On function which I can understand). Due to this limitation, any feat or item that wants to double speed has to have an iteration for EVERY SINGLE SPEED SETTING from that Heavy Armor wearing Halfling with a STR under 13 (or 15) having a speed of 15 to the unarmored Monk 15, Barbarian 5 Centaur with the Mobile feat wearing activated Boots of Speed having a speed 170 can be Hasted to double those numbers. I started making a table to keep track of this but just gave up when I realized how big an undertaking this has become.

Combine these two facts together and if I wanted to make a workable item for every single speed setting and extra attack setting, I would need 4 iterations of an item for every speed 15-100 (centaur+mobile+boots of haste+fighter) and 2 iterations for every speed 105-170.  I believe that is 90 different iterations of the same thing to reproduce desired effect of the Haste spell in an item that one can turn on and off with 1 mouse click.

This was incredibly comprehensive and extremely useful, THANK YOU! 

Thank you for your kind words. I have been considering publishing the more common speed levels and not even accounting for things like Boots of Speed or the heavy armor penalty because how often are either of those in play. And who uses a centaur?

Even then we're looking at 4 each for speeds 25-45 and 2 each for speeds 45-75 for a total of 34 slightly different items. I'm not sure how the community would feel about 34 slightly different Haste spell effects filling up the homebrew screen. To be fair though, a person only needs 1 in their inventory at a time.