1. Create a shortcut of the Shadow Era application (not the launcher) and place it somewhere easy like your desktop. By default, it is located here:
C:\Users\<name>\AppData\LocalLow\Wulven Game Studios\Shadow Era Launcher\game\v3.52100\ShadowEra.exe

2. Right-click the shortcut and click Properties.

3. Edit the Target by adding -popupwindow to the end of it. For example:
"C:\Users\<name>\AppData\LocalLow\Wulven Game Studios\Shadow Era Launcher\game\v3.52100\ShadowEra.exe" -popupwindow

4. Select Maximized under Run then Apply the changes and click OK.

You are now running the game in a beautiful fullscreen setup! Keep in mind that the launcher does not run with this method. So if there is a new version, you will have to open the launcher for the new version to be installed.

Then, you will have to create a new shortcut and follow the steps above to run the new game version in fullscreen.