1. Launch the game once through Steam and configure all your settings how you like them to be.

2. Close the game.

3. Open the following directory in File Explorer:

Windows: C:\Users\{user}\AppData\Roaming\kingdom_rush_alliance

Mac: /Users/{user}/Library/Application Support/kingdom_rush_alliance (thanks to Andyryw for the location)

4. Open the settings.lua file in a text editor (notepad++ recommended).

5. Add the following values:

["key_hero_1"] = "space";
["key_hero_2"] = "4";
["key_pow_1"] = "2";
["key_pow_2"] = "3";
["key_pow_3"] = "1";
["key_wave"] = "x";

Obviously when I say "following values" you can put in whatever you want, this is just what I use. Space for primary hero, 4 for secondary hero, 2 for reinforcements (as god intended), 3 for first hero ability, 1 for second hero ability.
Key_wave is for calling the next wave early, I have that at W, but I added it here for completion sake.

Link for more detailed explanation (for frontiers, but the engine is the same so it works here):

https://github.com/cigumo/krli/wiki/Configuring-key-bindings

Link for key names:

https://love2d.org/wiki/KeyConstant