How to link an amount from a text input to a payment button field later on in process.

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HTML

PHP

Hi people.
Been doing some updates to my website and got abit stuck.
I have a dropdown where a user selects payment amount "youhave":

<input type="text" name="youhave" class="form-control" value="1" id="youhave" value="1"

Then later on in the processing phase of my site i have some html buttons for perfectmoney, and advcash,
On these buttons there is a text field where you have to confirm the amount again before you click the appropriate payment button:

<form target="_blank" action="https://perfectmoney.is/api/step1.asp" 
method="POST">
<input type="hidden" name="PAYEE_ACCOUNT" value="U3492039">
<input type="hidden" name="PAYEE_NAME" value="Cryptonage">
<input type="hidden" name="PAYMENT_ID" value="Cryptonage">
<input type="text"   name="PAYMENT_AMOUNT" value="">               
<input type="hidden" name="PAYMENT_UNITS" value="USD">
<input type="hidden" name="STATUS_URL" 
value="http://127.0.0.1/cryptonage.net">
<input type="hidden" name="PAYMENT_URL" 
value="http://127.0.0.1/cryptonage.net">
<input type="hidden" name="PAYMENT_URL_METHOD" value="GET">
<input type="hidden" name="NOPAYMENT_URL" 
value="http://127.0.0.1/cryptonage.net">
<input type="hidden" name="NOPAYMENT_URL_METHOD" value="GET">
<input type="hidden" name="SUGGESTED_MEMO" value="">
<input type="hidden" name="BAGGAGE_FIELDS" value="">
<input type="submit" name="PAYMENT_METHOD" value="Pay with PerfectMoney">
</form>

how can i link the amount that was entered in "youhave" to the "PAYMENT_AMOUNT" input and hide the text field instead of having to type the amount again in the text field next to the button?

thanks in advance, updated question and provided code.

What I have tried:

changing text input for amount to hidden and setting value to "youhave", not mega experienced just trying to resolve this small issue.