SilverStripe functional testing for CMS route (settings) always returning 404 for POST request - silverstripe

Question

I am working on a SilverStripe project. I am writing functional tests for my application. I am writing a function test for a path within the CMS. In my case the path is /admin/settings/SecondaryMenuPages. It is the path within the CMS and so the user needs to be logged in.
This is my test class.
class MegaMenuTest extends FunctionalTest
{
protected static $fixture_file = [
'app/tests/Cms/cms_admin_user_fixture.yml',
'app/tests/Cms/secondary_menu_fixture.yml',
];
function testCmsUserCanUnlinkSecondaryMenuItem()
{
$member = $this->objFromFixture(Member::class, 'admin_user');
$adminGroup = $this->objFromFixture(Group::class, 'admin_group');
$this->assignMemberToGroup($member, $adminGroup);
$this->assignPageToSecondaryMenu();
$this->logInAs($member);
$this->post("/admin/settings/SecondaryMenuPages", [
'ID' => 1,
], null, null, json_encode([ 'ID' => 1 ]));
}
private function assignMemberToGroup($member, $adminGroup)
{
$sqlInsert = SQLInsert::create('Group_Members');
$sqlInsert->addRows([
[ '"GroupID"' => $adminGroup->ID, '"MemberID"' => $member->ID ],
]);
$sqlInsert->execute();
}
private function assignPageToSecondaryMenu()
{
$page = $this->objFromFixture('Page', 'page_one');
$siteConfig = $this->objFromFixture(SiteConfig::class, 'siteconfig_one');
$sqlInsert = SQLInsert::create('SiteConfig_SecondaryMenuPages');
$sqlInsert->addRows([
[ '"PageID"' => $page->ID, '"SiteConfigID"' => $siteConfig->ID ],
]);
$sqlInsert->execute();
}
}
Literally, the page is under the settings within the CMS.
I seeded the admin user in the cms_admin_user_fixture.yml file as follows.
SilverStripe\Security\Member:
admin_user:
ID: 1
LastEdited: 2020-03-28 17:15:53
Created: 2019-01-08 17:04:53
FirstName: Default Admin
Email: admin
SilverStripe\Security\Group:
admin_group:
ID: 1
LastEdited: 2019-01-08 17:04:53
Created: 2019-01-08 17:04:53
Title: Administrators
Code: administrators
Locked: 0
I seeded the page and required data for request in the secondary_menu_fixture.yml file as follows.
SilverStripe\CMS\Model\SiteTree:
sitetree_page_one:
ID: 1
ClassName: Page
Title: Page Title 1
URLSegment: page-url-1
Page:
page_one:
ID: 1
Heading: Page Title 1
Title: Cision form content
URLSegment: page-url-1
SilverStripe\SiteConfig\SiteConfig:
siteconfig_one:
ID: 1
LastEdited: 2020-03-10 12:25:37
Created: 2019-01-08 17:04:53
Title: Siteconfig Title 1
Tagline: Siteconfig Tagline 1
CanViewType: Anyone
CanEditType: OnlyTheseUsers
CanCreateTopLevelType: OnlyTheseUsers
When I run my test, the post request to this path, "/admin/settings/SecondaryMenuPages" is always returning 404. What is wrong with my code and how can I fix it?