You can set the text of a RichTextBox control by simply doing:
richtextbox1.Text =form1.textbox1.text
Or, you can use the AppendText method:
richtextbox1.AppendText(form1.textbox1.text)
This is the kind of thing you can easily learn on your own if you use the Intellisense in Visual Studio. If you type RichTextBox1 and a dot "." then you get a list of all the properties and methods and should be able to see it pretty easily.
https://msdn.microsoft.com/en-us/library/system.windows.controls.richtextbox(v=vs.110).aspx[
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