Boundary in multipart form-data to HTTP Request

Hello, to perform a POST type integration in which I want to add a content type multipart/form-data to the body, I have to add the boundary field to the integration header, which I have seen that appian generates in the HTTP Request .

That value that is generated in the HTTP Request is --TnQmfygKfP801A_kE1bGL-7YAFk_k6gl9--, and after testing in different environments and files, I have seen that it is always the same.

POST /api/myit-sb/content HTTP/1.1
Content-Type: multipart/form-data; boundary=TnQmfygKfP801A_kE1bGL-7YAFk_k6gl9
X-Requested-By: XMLHttpRequest
Cache-Control: no-cache
Content-Disposition: form-data; name="file"; filename="OP_ATP_DocPrueba.docx"
Content-Length: 12354
Host: aena-dev-restapi.onbmc.com
Connection: Keep-Alive
User-Agent: Appian
Accept-Encoding: gzip,deflate

--TnQmfygKfP801A_kE1bGL-7YAFk_k6gl9
Content-Disposition: form-data; name="file"; filename="OP_Test_DocPrueba.docx"
Content-Type: application/vnd.openxmlformats-officedocument.wordprocessingml.document
Content-Transfer-Encoding: binary

Binary content not shown for document, OP_Test_DocPrueba.docx (11.79 KB).
--TnQmfygKfP801A_kE1bGL-7YAFk_k6gl9--

Can someone tell me how to capture the value --TnQmfygKfP801A_kE1bGL-7YAFk_k6gl9-- in the HTTP Request or tell me if that value is always the same for Appian?

Thank you so much.