How do I prompt the user for "Device Administrator" on Tasker?
I'm no expert on intents but this is the best answer I can offer...

There are intents that can enable the Device Admin status but since Tasker is not yet a device admin I don't think your prompt will allow the user to enable this directly.   

The next best thing would be to have Tasker open the Device Administration settings page with a popup that instructs them to enable it.  

In tasker, using the "Settings Dialog Action" you can create an action that opens to the Security Settings page but the user would have to manually go to the next step to open the "Device Administror" menu.  You can create a popup message directing what to do with the "Alert/Popup Action".

If you want to go directly to the Device Administror Menu you may be able to do that with the "Misc/Send Intent Action."  To find specific intent for the Device Admin Menu you can try an app called Inent Interceptor.  If you are able to identify the specific intent that opens that menu it's pretty straight forward from there.