Go

package main

import (
	"bytes"
	"fmt"
	"io"
	"mime/multipart"
	"net/http"
	"os"
)

func main() {
	var body bytes.Buffer
	w := multipart.NewWriter(&body)

	f, _ := os.Open("applicant.pdf")
	defer f.Close()
	part, _ := w.CreateFormFile("file", "applicant.pdf")
	io.Copy(part, f)
	w.WriteField("detect", "fields")
	w.Close()

	req, _ := http.NewRequest("POST", "https://api.createafillablepdf.com/v1/convert", &body)
	req.Header.Set("Authorization", "Bearer "+os.Getenv("CAFP_KEY"))
	req.Header.Set("Content-Type", w.FormDataContentType())

	res, err := http.DefaultClient.Do(req)
	if err != nil {
		panic(err)
	}
	defer res.Body.Close()
	out, _ := io.ReadAll(res.Body)
	fmt.Println(res.StatusCode, string(out))
}

Fill with POST /v1/fill (JSON body { "conversion_id": "...", "values": { ... } }).