Summary
This online utility takes an HTTP GET request, converts it to an HTTP POST request, and sends it to the proper Web server.
What are GET and POST requests?
GET and POST are the two standards to send web forms, for doing searches or extraction of information from a database. The difference is discussed at W3School's HTTP methods GET vs POST, section Compare GET vs. POST.
Why HTTP POST Relayer?
In brief: HTTP POSTs cannot be saved in browser history or bookmarks, contrary to HTTP GETs. Usually online forms use GET: the search URL holds the input parameters. When you face a POST form you see that the landing page, after submission, has no signs of the query inputs. You cannot save the query for later use or for sharing it with other people.
HTTP POST Relayer makes it possible. It takes a special GET request/URL (with the query data visible) reshapes it as a POST request, and sends it to the needed site. The target Web server is unaware of this trick and returns the correct reply page.
The main advantage is given by the direct use operation (read below) allowing a normal link to hold all the query arguments while the final request will still be sent as POST.
How to create working / shareable POST requests
There are two way of operation: manual and direct. In both cases you should know the field names and the values you want to apply to them. Modern browsers allow to inspect web pages and grab the needed information. To avoid the hassle of digging into html you can install the bookmarklet PostToGet in your browser. Go to the page containing the (POST) query form and fill in the fields. Don't send the form, but instead click the PostToGet bookmarklet to obtain a link written in GET format. If the page holds multiple forms you'll get a link for each form. Please refrain to work with forms containing private information / usernames / passwords, because they become visible and unsafe to be sent on the net.
Manual use
Fill the URL field below formatted as a normal GET request. Follow the suggestion or paste the result given by above bookmarklet. Press Send to fire immediately your query as POST. For sending the query later, just press Copy. What's copied to the clipboard is not the URL you've inserted, but a direct use link. It is "ready to work": you don't need to stop again on this page and write or paste anything by hand. A direct link works by itself, as any other normal link. It can be shared, written down in texts and references, or saved as a bookmark in your browser.
Direct use
By hand: append the URL to be converted from GET to POST just after the URL of this page.
The data to be added are: address of the query page (where POST query should go) followed by the list of query field names with their values.
The exact syntax is a sort of "double GET" request: + ? + query page location including path + ? + arguments (name1=value1&name2=value2&...).
How direct links work?
Direct links are received here, on this page. Whenever query arguments exist they are processed here, on this page. A true POST form is reconstructed with all its fields and values. Finally it is automatically fired. All the process occurs on the page, in your browser, not at the server level. Nonetheless the server (where this page is hosted) will see your query values. That's the reason for not using this page for processing sensitive or private data.
Bonus
A direct link allows to archive a page even when generated by a POST request. This is not trivial. Archive.org cannot save pages related to POST requests. On the contrary archive.today, currently at https://archive.ph/, succeeds. You need to feed it with the direct link. The archive.today system performs the redirection (from GET to POST), lands on the correct POST page and archives it.
Acknowledgements
Most of this work is based and/or inspired by the page GET to POST now defunct and accessible only as an archived page on the Wayback machine.