In ArcGIS for Server 10.3, passing the username and password credentials in plain text to generate tokens through a HTTP Get request is not possible. The link provided below is of a documentation which is worded in a way that confuses customers about this workflow:
Even if Get requests are enabled via the documentation, they are still unusable with credentials, which is pointless to enable them at all. It also contradicts another documentation (link provided below) that explicitly states that passing credentials in plain text through Get requests is not possible.
Last Published: August 6, 2015ArcGIS for Server
Bug ID Number
March 23, 2015
November 4, 2023
ArcGIS for Server
Operating System Version
7 64 Bit
The bug has been fixed. See the Version Fixed and Additional Information, if applicable, for more information.