I've built a custom alert action with a UI. One of my inputs is dynamic, and populated from a splunk search. Here is the markup in custom alert HTML file.
I need the ability to enter a custom value as well, but allow-custom-value has no impact. According to this code sample it should work: https://docs.splunk.com/Documentation/Splunk/6.5.2/AdvancedDev/CustomAlertUI#Dynamic_input_controls
search="host=things| sort thingid | table thingid thingnum| dedup thingid"
This appears to be a bug and not working in version 7.2.1 and maybe other versions. What version are you running?
If you look at the bottom of this page: https://docs.splunk.com/Documentation/Splunk/latest/AdvancedDev/CustomAlertUI you can see others have reported it too. You should raise a support ticket to see if support can give you a workaround. I can't find any mention of this bug on the known issues page so it must be pretty new.
All the best.
I tried but it looks like I don't have a support contract.