If your are using Mendix Windows Service 4 or higher, go to the location for the apps and server files (Press button Preferences on the toolbar to view this location).
Open the Apps folder and then the folder of your app. The file Settings.yaml is located in that folder. In this file, go to the setting Runtime, ServerPassword; that is the M2EE password. Convert this password to base64 and put it in the request header.
I know it has been 3 years since the question was posted but I wanted to share my findings on the topic.
My situations was: I was running a project from the modeler locally. I could not find a place to set the m2ee admin pass. I could not find any settings.yaml file as suggested in the accepted answer. Luckily, I stumbled upon this helpful little post http://www.mxblog.eu/p/post/2533274790398005.
With the help of process explorer I was able to get the auto-generated m2ee password and succsessfully run my monitoring POST requests.
Hurray and thanks to the original author of the post and all people who provided their inisght,
It's your M2EE password, not a user password. Where are you running exactly? On linux, this is stored in your m2ee.yaml file.
I'm running on a Windows 2008 sever.
I looked for a m2ee.yaml file but I could not find one.
Where is this M2EE password stored on Windows ?