How to transfer property from response to endpoint parameter

I have a request with one parameter:

This parameter is in a response from another request (the number that can be seen in the transfer log).

As I can see the value in the log I guess that the Source part is covered, but I have not idea how to get the Target part right. I cannot select the property as a target.

I tried concatenating the whole endpoint and the response - Source : concat('my_full_rest_url/',$..*) - and setting Target Property to Endpoint, but it results in Invalid JSON. My response is only plain number : 12334576.

Help?