I was trying to call an external service using ajax internal implementation of SAP IRPA but it wasn't working. I realize that the implementation of ajax asumes that content type application/x-www-form-urlencoded means the data has to be sent in the url, which I think it is wrong.
The code can be found in the Ctx Languaje internal library, inside the ajax definition around line 2347. There should be another special case when options.contentType is equal to e.ajax.content.form (application/x-www-form-urlencoded), in which the data should be passed to _buildQuery but stored in body.data. Also I think the size should be the size in bytes, not necesary the length of the string
Hope it helps, cheers.