Where to find player needs Your Heroes help request?

I have been told that the button to request help from other Alliance member’s Heroes is supposed to be on the same screen as the team-ups. however, I have never seen it, even when I have 19 or fewer team ups.
I am playing on Android if that helps in case it’s a platform specific option.

You’re looking for the “get team-ups from your alliance” button at the bottom.

(Sorry for the image size, it’s the best one I found online.)

If that’s it, then how does one get it to request Heroes’ help instead of team ups?

What is “Heroes’ help?”

“[Alliance member] needs your heroes’ help” message above Alliance chat.

When you hit the 'get team-ups from your alliance’ button then the ‘…needs your heroes’ help message is automatically added to the alliance chat. This is where you select characters in your roster to send as team-ups

The several times I have tried this, it has always requested team-ups rather than heroes. Does the game randomly request between the two or is there some other factor involved?

They are the same thing. You’ll notice in that same screenshot, beside the yellow button, the phrase “0/5 Team-Ups Sent.” Team-ups, heroes’ help, all the same thing.

To put it another way, in the context of the game, your heroes help an ally by loaning them their powers in the form of Team-Ups.