Can't set a test license token in an Outlook addin

I'm trying to set a test license token in an Outlook add-in. According to the documentation, I have to set it in the SourceLocation in the manifest file:

<Form xsi:type="ItemRead">
  <DesktopSettings>
    <SourceLocation DefaultValue="https://localhost:3000?et=%3Cr%3E%0A%20%20%3..."/>
    <RequestedHeight>250</RequestedHeight>
  </DesktopSettings>
</Form>

The problem is, when I try to get the URL query params, it won't work. window.location looks like this:

https://localhost:3000/?et=

Any idea why it's happening?

When I update the source location of the Action ShowTaskpane, it won't start the addin:

<Action xsi:type="ShowTaskpane">
    <SourceLocation resid="messageReadTaskPaneUrl" />
</Action>

...

<bt:Url id="messageReadTaskPaneUrl" DefaultValue="https://localhost:3000?et=%3Cr%3E%0A%20%20%...">

If I set it like this, when I start the addin, it says:

Sorry, but we can't start this add-in because it isn't set up properly.

I use the following test token:

<r>
  <t 
    aid="WA907006056" 
    pid="{4FB601F2-5469-4542-B9FC-B96345DC8B39}" 
    cid="32F3E7FC559F4F49" 
    did="{0672BAE9-B41B-48FE-87F1-7F4D3DD3F3B1}" 
    ts="30" 
    et="Trial" 
    ad="2012-01-12T21:58:13Z" 
    ed="2019-06-30T21:58:13Z" 
    sd="2012-01-12T00:00:00Z"
    test="true" 
    te="2019-06-30T02:49:34Z" />
  <d>VNNAnf36IrkyUVZlihQJNdUUZl/YFEfJOeldWBtd3IM=</d>
</r>

And use the following service to encode the url:

https://www.urlencoder.org/