public override async Task GrantResourceOwnerCredentials(OAuthGrantResourceOwnerCredentialsContext context)
        {
           //here you get the context.UserName and context.Password
           // and validates the user
        }

$.ajax({
            type: 'POST',
            url: Helper.ApiUrl() + '/token',
            data: { grant_type: 'password', username: UserName, password: Password },
            success: function (result) {
                Helper.TokenKey(result.access_token);
                Helper.UserName(result.userName);
            },
            error: function (result) {
                Helper.HandleError(result);
            }
        });

data: { grant_type: 'password', username: UserName, password: Password, customer: Customer }

//here you get the context.UserName, context.Password and context.Customer

var customer = context.Parameters.FirstOrDefault(x => x.Key == "customer").ToString()).Value;

public override async Task GrantResourceOwnerCredentials(OAuthGrantResourceOwnerCredentialsContext context)
        {
            //here you read all the params
            var data = await context.Request.ReadFormAsync();
            //here you get the param you want
            var param = data.Where(x => x.Key == "CustomParam").Select(x => x.Value).FirstOrDefault();
            string customer = "";
            if (param != null && param.Length > 0)
            {
                customer = param[0];
            }

}

data: { grant_type: 'password', username: user, password: pwd, CustomParam: 'MyParam' },