Press enter key on the textbox and do the click event of the button

Hi everybody, sorry for my English but I�´ll do as well as I can.......

I have a text box and a button and when I press the enter key in the text box I want to do the click event of the button...

I�´ve done this but when I�´m going to send the instruction to the server with "RegisterStartupScript" I don�´t know what I have to put between the brackets:

 

Private Sub txtParametro_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles txtParametro.TextChanged

Dim Clientscript As String = "<script>btnMostrar_Click</script>"

RegisterStartupScript(, Clientscript)

End Sub

 

I don�´t know if it�´s correct, can someone help me? please, I�´ve been searching how to do it but I don�´t find anywhere...

For example when I make a popup whith JavaScript I put: RegisterStartupScript(" ...