I follow the reference here "https://github.com/biouno/uno-choice-plugin/wiki/Using-Uno-Choice-for-Dynamic-Input-Text-Box-Defaults" to pass Reactive Reference Values to the build.

I have 3 Active-Choice parameters:
1.
refer to Selection_003.png
Active Choices Parameter: URL_ROOT

return [ "https://www.google.ca" ]

2.
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Active Choices Reactive Parameter: SUB_PATH

return [ "test1", "test2", "test3" ] 

3.
refer to Selection_005.png
Active Choices Reactive Reference Parameter (Formatted HTML): COMBINED_URL

return """<input name="value" value="${URL_ROOT}/${SUB_PATH}" class="setting-input" type="text" readonly>"""

When triggering the build with parameters,
the triggering page is displayed everything correctly.
HTML code for Formatted HTML:

<input class="setting-input" type="text" readonly="" value="https://www.google.ca/test2" name="value">

Refer to Selection_001.png and Selection_002.png

However,
during the build,
Jenkins will get an extra comma character ',' at the end.

COMBINED_URL=https://www.google.ca/test1,

Refer to Selection_006.png

And Jenkins parameter page also confirm an extra comma character at the end.
Refer to Selection_007.png