curl --request POST \
  --url 'https://{domain}/wp-json/sagescreen/v1/wp-admin/admin-ajax.php?nonce=&action=sagescreen_mark_splash_seen' \
  --header 'Content-Type: application/x-www-form-urlencoded' \
  --data action=sagescreen_mark_splash_seen \
  --data 'nonce=<string>' \
  --data splash_id=123

import requests

url = "https://{domain}/wp-json/sagescreen/v1/wp-admin/admin-ajax.php?nonce=&action=sagescreen_mark_splash_seen"

payload = {
    "action": "sagescreen_mark_splash_seen",
    "nonce": "<string>",
    "splash_id": "123"
}
headers = {"Content-Type": "application/x-www-form-urlencoded"}

response = requests.post(url, data=payload, headers=headers)

print(response.text)

const options = {
  method: 'POST',
  headers: {'Content-Type': 'application/x-www-form-urlencoded'},
  body: new URLSearchParams({action: 'sagescreen_mark_splash_seen', nonce: '<string>', splash_id: '123'})
};

fetch('https://{domain}/wp-json/sagescreen/v1/wp-admin/admin-ajax.php?nonce=&action=sagescreen_mark_splash_seen', options)
  .then(res => res.json())
  .then(res => console.log(res))
  .catch(err => console.error(err));

<?php

$curl = curl_init();

curl_setopt_array($curl, [
  CURLOPT_URL => "https://{domain}/wp-json/sagescreen/v1/wp-admin/admin-ajax.php?nonce=&action=sagescreen_mark_splash_seen",
  CURLOPT_RETURNTRANSFER => true,
  CURLOPT_ENCODING => "",
  CURLOPT_MAXREDIRS => 10,
  CURLOPT_TIMEOUT => 30,
  CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
  CURLOPT_CUSTOMREQUEST => "POST",
  CURLOPT_POSTFIELDS => "action=sagescreen_mark_splash_seen&nonce=%3Cstring%3E&splash_id=123",
  CURLOPT_HTTPHEADER => [
    "Content-Type: application/x-www-form-urlencoded"
  ],
]);

$response = curl_exec($curl);
$err = curl_error($curl);

curl_close($curl);

if ($err) {
  echo "cURL Error #:" . $err;
} else {
  echo $response;
}

package main

import (
	"fmt"
	"strings"
	"net/http"
	"io"
)

func main() {

	url := "https://{domain}/wp-json/sagescreen/v1/wp-admin/admin-ajax.php?nonce=&action=sagescreen_mark_splash_seen"

	payload := strings.NewReader("action=sagescreen_mark_splash_seen&nonce=%3Cstring%3E&splash_id=123")

	req, _ := http.NewRequest("POST", url, payload)

	req.Header.Add("Content-Type", "application/x-www-form-urlencoded")

	res, _ := http.DefaultClient.Do(req)

	defer res.Body.Close()
	body, _ := io.ReadAll(res.Body)

	fmt.Println(string(body))

}

HttpResponse<String> response = Unirest.post("https://{domain}/wp-json/sagescreen/v1/wp-admin/admin-ajax.php?nonce=&action=sagescreen_mark_splash_seen")
  .header("Content-Type", "application/x-www-form-urlencoded")
  .body("action=sagescreen_mark_splash_seen&nonce=%3Cstring%3E&splash_id=123")
  .asString();

require 'uri'
require 'net/http'

url = URI("https://{domain}/wp-json/sagescreen/v1/wp-admin/admin-ajax.php?nonce=&action=sagescreen_mark_splash_seen")

http = Net::HTTP.new(url.host, url.port)
http.use_ssl = true

request = Net::HTTP::Post.new(url)
request["Content-Type"] = 'application/x-www-form-urlencoded'
request.body = "action=sagescreen_mark_splash_seen&nonce=%3Cstring%3E&splash_id=123"

response = http.request(request)
puts response.read_body