Without more detail I am assuming that you are just needing to define a specific header to pass when making a request. You should be able to define that like this https://www.soapui.org/soap-and-wsdl/headers-and-attachments.html#1-Custom-HTTP-Headers
I'm not trying to add a custom header to the request, I'm trying to customize the Content-Disposition header that is generated for each attached file.
When I upload multiple files, SoapUI generates a Content-Disposition header that contains, among other things, the name of the file. I'm working with a web service that expects custom content.
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Disposition
POST /test.html HTTP/1.1 Host: example.org Content-Type: multipart/form-data;boundary="boundary" --boundary Content-Disposition: form-data; name="field1" value1 --boundary Content-Disposition: form-data; name="field2"; filename="example.txt" value2 --boundary-- Specifications
