Your logic and program flow is a bit mangled.

Try this to start with, when you have it working ask questions about why it is structured in the way that it is:
 Code:
Break on
$=SetOption("NoVarsInStrings","ON")

$PROJEKT = createobject("KIXTART.form")
$PROJEKT.backcolor = &2CCD148
$PROJEKT.text = "Textbox"
$PROJEKT.width = 660 
$PROJEKT.height = 660
$PROJEKT.show

; Create Input Box
$Input_1 = $PROJEKT.controls.textbox
$Input_1.left = 430
$Input_1.top = 90
$Input_1.width = 70
$Input_1.FontSize = 14
$Input_1.SetFocus

; Create action button
$My_Button = $PROJEKT.controls.button("Calculate")
$My_Button.top = 250
$My_Button.left = 175
$My_Button.width = 250
$My_Button.FontSize=14

; Assign action to button
$My_Button.onclick = "f_Eingabe($INPUT_1.text)"



while $PROJEKT.visible
	$=execute($PROJEKT.doevents)
Loop
Exit 0

Function f_Eingabe($E_Text)
	$Projekt.messagebox($E_Text,$E_Text,0)
Endfunction