undifined function name in custom plugin

Hi, I am creating a custom form plugin that in the method="POST" will go to specific .php file to do some function but it seems the WP Frameworl/Functions didn’t call or didn’t define.

this is my code in my custom plugin myform.php

function SRMS_form_func( ) {
	//require_once ( plugins_url( 'srmsform_post.php', __FILE__ ));
	$form_ret = "<form action='".plugins_url( 'srmsform_post.php', __FILE__ )."' method='post'>";
	$form_ret .= "</br><label>First Name</br></label><input type='text' name='givenName' value=''></br>";
	$form_ret .= "</br><label>Last Name</br></label><input type='text' name='surname' value=''></br>";
	$form_ret .= "</br><input type='submit' value='Submit'>";
	$form_ret .= "</form>";

	return $form_ret;
}
add_shortcode( 'SRMS_form', 'SRMS_form_func' );

in my srmsform_post.php is this.

<?php
  $srmshost = get_option('srmshost');

in this line I always get an error of
Fatal error: Call to undefined function get_option()

how can I define functions in WP in srmsform_post.php when myform.php calls the POST method?