Hi,
Fusion 360 currently does not support to add external text commands. It is able to run python script in Text Commands window. So one workaround, you may write your text commands in python script and run python script in Text Commands window. For example:
1. Create a python script named "MyTextCommands" with the following content.
import adsk.core, traceback
def showMessageBox(msg):
ui = None
try:
app = adsk.core.Application.get()
ui = app.userInterface
ui.messageBox(msg)
print('Message: ' + msg)
except:
if ui:
ui.messageBox('Failed:\n{}'.format(traceback.format_exc()))
2.Show text commands window, choose radio "py".
3.Add the folder path of "MyTextCommands.py" to sys.path:
Enter "import sys" in text commands window.
Enter "sys.path.insert(0, "C:/Users/liujac/AppData/Roaming/Autodesk/Autodesk Fusion 360/API/Scripts/MyTextCommands")" in in text commands window.
4.Enter "import MyTextCommands" in text commands window.
5.Enter "MyTextCommands.showMessageBox("Hello World")" in text commands window. The message box is displayed and the message is print in text commands window.

I'm not sure if this is exactly what you expected. It would be great if you can share more detailed info about your workflow, that would be helpful for us to understand the requirements.
Regards,
Jack