The offical MS documentation on this field:
The URL field uniquely consists of two strings that are separated by a comma and space. One string contains the URL path and the other contains the description that is used as hyperlinked text.
So your REST call should look like this:
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REPEAT: There is a space between the comma and the description.
If you’re interested in learning how to save other field types, leave me a comment and I’ll write something up.