试图在表单提交上调用外部php文件，获取404错误

I have a form which is being echoed by php:

echo '<form action="stsupport.php" method="POST">
<p>
Priority Name: <input type="text" size="50" name="ticketpriority" placeholder="Input a name for your priority..."></textarea>
</p>

<input type="submit" name="submitpriority" value="submit">

</form>';

I want the stsupport php file to run when the form is submitted but I am getting 404 error when the form is submitted.

The stsupport.php file is in the same folder as the file with the form.

Any ideas?

Edit:

This is in the support.php file where i call the function to create priority. The create priority function just displays the form above.

require_once(ABSPATH . 'wp-content\plugins\stsupport\adminmenu.php');

create_priority();

When the form is submitted i want to run:

function st_support(){
    require_once(ABSPATH . 'wp-content\plugins\stsupport\adminmenu.php');

    create_priority();

    if (isset($data['submitpriority'])) {
        insert_priority();
    }
}

The insert_priority function is inside the adminmenu.php file

Try open stsupport.php via browser

Check your htaccess rule, if exist

Check file permission

Or

if(file_exists('stsupport.php')){
    $action = 'stsupport.php';
}else{
    $action = '';
}
echo '<form action="'. $action .'" method="POST">
<p>
Priority Name: <input type="text" size="50" name="ticketpriority" placeholder="Input a name for your priority..."></textarea>
</p>

<input type="submit" name="submitpriority" value="submit">

</form>';

Try to type the the full directory of the root folder in the form. also set the name of the form as the name attribute in the input tag.
'<form name='submitpriority' action="../DIRECTORY/stsupport.php" method="POST">