
function get_val( tot_val )
{
document.getElementById('TextBox1').value = tot_val;
}
<tr>
<td><textarea id="TextBox1" name=textarea1 readonly="readonly" rows="5" cols="20">

Here is a simple code that insert some text into textarea on clicking the button.
<html>
<script>
function get_val( tot_val )
{
document.getElementById('TextBox1').value = tot_val;
}
</script>
<table>
<tr>
<td><textarea id="TextBox1" name="textarea1" readonly="readonly" rows="5" cols="20">

print(" function get_val( tot_val )
{
document.getElementById('TextBox1').value = tot_val;
}
<tr>
<td><textarea id="TextBox1" name=textarea1 readonly="readonly" rows="5" cols="38">
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