Here's an example URL to create a task, assign it to person, and enter a description:
https://tfs.example.com/tfs/TopLevel/MediumLevel/Project/_workItems/create/Task?%5BSystem.AssignedTo%5D=Wiley+Coyote&%5BSystem.Description%5D=%3Cp%3EExample+Description+Text.%3C%2Fp%3E
This looks like garbage because it is html encoded. Plain text, courtesy of www.utilities-online.info/urlencode/ would be:
https://tfs.example.com/tfs/TopLevel/MediumLevel/Project/_workItems/create/Task?[System.AssignedTo]=Wiley+Coyote&[System.Description]=Example+Description+Text.
This will create a task, but not save it.
Here is a non-trivial example, unescaped, and put on multiple lines for clarity:
https://tfs.example.com/tfs/TopLevel/MediumLevel/Project/_workItems/create/User Story?[System.AssignedTo]=Wiley+Coyote
&[Acme.ENABLE.TRAP.ID]=797
&[Acme.ENABLE.PROJ.ID]=777
&[Microsoft.VSTS.Common.ValueArea]=Business
&[Microsoft.VSTS.Common.AcceptanceCriteria]=The Roadrunner is captured.
&[System.Tags]=Food
&[System.AreaPath]=Plans\Trap
&[System.Title]=Giant Mouse Trap to Catch Roadrunner
&[System.Description]=Order giant mouse trap
Place bird seed in trap
wait for roadrunner to get caught
Place bird seed in trap
wait for roadrunner to get caught
The tricky part is finding the names of the element you want to add. Fortunately, you can get a Visual Studio plugin which will allow you to see the standard element names and your companies custom names.
You can download the TFS Process Template Editor here.
It will install into VS2017 under the Tools menu. Select the Process Editor and the "Work Item Field Explorer".
This will display a list of the field names including your custom defined ones:
This method works will for small tasks, but for larger tasks, use the API.