We can create custom api for GraphQL.

As per the app/code/ standard, our module name is Magemonkeys/Customgraphql

We can create schema.graphqls file in module etc folder.

Below is the code for that file.

type Query
{
    CustomGraphql (
        username: String @doc(description: "Email of the customer")
        password: String @doc(description: "Password")
        fieldtype: String @doc(description: "Field Type")
    ): CustomGraphqlOutput @resolver(class: "Magemonkeys\Customgraphql\Model\Resolver\ResolverGraphql") @doc(description:"Customgraphql Datapassing")
}
type CustomGraphqlOutput
{
    username: String
    password: String
    fieldtype: String
}

After that we need to create Model Resolver file.

Create file ResolverGraphql.php in this location of module
Magemonkeys/Customgraphql/Model/Resolver/

Below is the code for that file.

<?php
namespace MagemonkeysCustomgraphqlModelResolver;

use MagentoFrameworkGraphQlConfigElementField;
use MagentoFrameworkGraphQlExceptionGraphQlInputException;
use MagentoFrameworkGraphQlQueryResolverInterface;
use MagentoFrameworkGraphQlSchemaTypeResolveInfo;
use MagentoFrameworkExceptionNoSuchEntityException;
use MagentoFrameworkGraphQlExceptionGraphQlNoSuchEntityException;

class ResolverGraphql implements ResolverInterface
{
    public function resolve(
        Field $field,
        $context,
        ResolveInfo $info,
        array $value = null,
        array $args = null)
    {
        if (!isset($args['username']) || !isset($args['password']) || !isset($args['fieldtype'])||
            empty($args['username']) || empty($args['password']) || empty($args['fieldtype']))
        {
            throw new GraphQlInputException(__('Invalid arguments list please check.'));
        }
        $output = [];
        $output['username'] = $args['username'];
        $output['password'] = $args['password'];
        $output['fieldtype'] = $args['fieldtype'];
      
        return $output ;
    }
}

After that you can check API in Postman API tool.

That’s it.