The usage would be...
response = dashboard.wireless.getNetworkWirelessUsageHistory( network_id , t0=starttime, t1=endtime, ssid=netssid) <<<<edit: this won't work, the call also needs a specific client/device given, see post further down for alternatives
Note that the call defaults to a resolution of 86400 seconds (one day), the return is an array of time slices, if you choose a resolution smaller than the time period you request data for, you'll get multiple time slices returned, each for a different part of the overall period.
t0 and t1 are strings, giving respectively the start and end of the period you want data for, they can be ISO format timestamps, or simple epoch timestamps (see... https://www.epochconverter.com/ for a way to manually make these), if you want scripts to automatically step through a range of dates, using epoch timestamps can be simpler.
If you just want some simple way to get a specific day, such as 31st March you could hardcode them...
starttime = "2023-03-31T00:00:00Z"
endtime = "2023-04-01T00:00:00Z"
...this is an ISO format datetime with the Z specifying this is UTC, if you're not working in UTC you'd need to adjust things.
Doing the same with epoch timestamps would be...
starttime = str(1680217200)
endtime = str(1680217200 + 86400)
...note that you need to turn the integer into a string for the API call, and to get the end time I simply added 86400 seconds to the start time, much easier than creating the human readable format.
If you want to automatically generate timestamps, there're many ways Python can handle datetimes, best way to learn is make some simple script to get familiar, see... https://docs.python.org/3/library/datetime.html
netssid is an integer identifying the SSID
To find the SSIDs on a network, use the getNetworkWirelessSsids call...
https://developer.cisco.com/meraki/api-v1/#!get-network-wireless-ssids
...it returns an array of all your SSIDs.
Or you can find the SSID number manually in Dashboard, go to the wireless SSID page, display all SSIDs, the leftmost one is 0, the next is 1, then 2, etc., it's just a simple integer.