how to call keydown event on particular button click

Like this.
Not sure what you tried. But this complete code is generated by Visual Studio itself. All you have to do is hook the control(button in this case) to the respective events.

public Form1()
{
	InitializeComponent();
}

private void button1_KeyDown(object sender, KeyEventArgs e)
{

}

private void button1_KeyPress(object sender, KeyPressEventArgs e)
{

}

private void button1_KeyUp(object sender, KeyEventArgs e)
{

}

private void InitializeComponent()
{
	this.button1 = new System.Windows.Forms.Button();
	this.SuspendLayout();
	// 
	// button1
	// 
	this.button1.Location = new System.Drawing.Point(52, 27);
	this.button1.Name = "button1";
	this.button1.Size = new System.Drawing.Size(75, 23);
	this.button1.TabIndex = 0;
	this.button1.Text = "button1";
	this.button1.UseVisualStyleBackColor = true;
	this.button1.KeyDown += new System.Windows.Forms.KeyEventHandler(this.button1_KeyDown);
	this.button1.KeyPress += new System.Windows.Forms.KeyPressEventHandler(this.button1_KeyPress);
	this.button1.KeyUp += new System.Windows.Forms.KeyEventHandler(this.button1_KeyUp);
	// 
	// Form1
	// 
	this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);
	this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
	this.ClientSize = new System.Drawing.Size(292, 273);
	this.Controls.Add(this.button1);
	this.Name = "Form1";
	this.Text = "Form1";
	this.ResumeLayout(false);

}

Look at the correct answer by Manas.

I only want to add an improvement.
The "traditional" form of setting up a handler for the event is pretty bad. The code generated by Microsoft Designer is also bad and based on obsolete syntax. One bad thing is Microsoft violates its own naming rules (the do not recommend underscore character). I promote different approach which pays of soon. You should not click on events in Designer at all. Creation of them does not really accelerate your work but makes the code much less supportable.

More modern and supportable approach is based on anonymous delegates. Before showing you form, set up the events. I usually do it in form's constructor, at the end of it:

MyButton.KeyDown += delegate(object sender, KeyEventArgs eventArgs) {
    LogWrite(string.Format("KeyDown  - {0}", e.KeyData));
    //I also added Format, which is better then "+"
}

As you can see, it looks more simple and easier to right. Very often, you don't use one or both parameters of the handler. So, from withing the method body you can call any other method with parameters convenient for you, not for the event declaration :). This code is also more supportable and all under your control.

Now, chances are you're using C# v.3 or later. Than you can further simplify the code using lambda form of the delegate:

MyButton.KeyDown += (sender, eventArgs) => {
    LogWrite(string.Format("KeyDown  - {0}", e.KeyData));
    //I also added Format, which is better then "+"
}

In this form, you don't even need parameter types as they are automatically inferred from the event type. It's called type inference.

I think this style of code is also closer to what you meant in your Question.
That's it!

Good luck,

—SA

P.S.: I see the contradiction in the formulation of the question caused some argument on the source of event Button or Form.
You probably mean Button but code sample was about the Form.
The technique to be used is exactly the same for KeyDown, as Form is Conrtol. All what matters is what events are declared in what type.

You have to put code in to see if the key being pressed is the one you're interested in. Maybe you should review the info in MSDN:

http://msdn.microsoft.com/en-us/library/system.windows.forms.control.keyup.aspx[^]