Yea thats under "Map Info" > "Waves: Edit"

the wave definition is done by rules like:

From Wave X to Wave Y Spawn Z Enemies of Type U every V Wave (+ Shield and Spawn Count increase)

So if you want to change the first Wave (as in the starting wave) you could either add a new rule that spawns something in Wave 1 or change an old rule that effects wave 1 (by that you will probably also change the upcoming waves tho)

here is a screenshot of the menu you are looking for:

https://i.imgur.com/QcPlYlx.png

The wave timer (time between waves) is under "Map Info" > "Rules: Edit" btw

ESC -> Map Info -> Waves: Edit.

Edit: oopsie