Looping API calls

Hi NicolaiW ,
if you turn your list of customer numbers into a table, you can add a column with the following formula:

Web.Contents("https://webpageAPI.com/customers/" & [PageColumName] & "/contacts")

However, while this would work in Excel and Power BI Desktop, you'd get a dynamic data source error when you try to refresh this in the service.
There, you would have to modify like so:

Web.Contents("https://webpageAPI.com/customers",
[RelativePath= [PageColumName] & "/contacts")

Check out this article for more details about it: 
Chris Webb's BI Blog: Using The RelativePath And Query Options With Web.Contents() In Power Query And Power BI M Code (crossjoin.co.uk)