In the below requeststring, I have to pass one value from the post form data.

For e.g: I have a user name text box and I need to pass that user name value on form submission in the below request string:

String abc = post data from form;

String requestString = '<?xml version="1.0" encoding="UTF-8" standalone="yes"?> <bns:EnvironmentExtensions xmlns:bns="http://api.platform.boomi.com/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" id="XXXXXXXXXXXXXXXX" extensionGroupId="" environmentId="XXXXXXXXXXXXXXX"> <bns:connections> <bns:connection name="SFDC connection" id="XXXX"> <bns:field componentOverride="false" usesEncryption="false" encryptedValueSet="false" value="email@gmail.com" id="user"/> <bns:field componentOverride="false" usesEncryption="true" encryptedValueSet="true" id="password" value="password"/> </bns:connection> </bns:connections> </bns:EnvironmentExtensions>';


What i Need?
To pass abc string value in the body.

String requestString = '<?xml version="1.0" encoding="UTF-8" standalone="yes"?> <bns:EnvironmentExtensions xmlns:bns="http://api.platform.boomi.com/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" id="XXXXXXXXXXXXXXXX" extensionGroupId="" environmentId="XXXXXXXXXXXXXXX"> <bns:connections> <bns:connection name="SFDC connection" id="XXXX"> <bns:field componentOverride="false" usesEncryption="false" encryptedValueSet="false" value= abc id="user"/> <bns:field componentOverride="false" usesEncryption="true" encryptedValueSet="true" id="password" value="password"/> </bns:connection> </bns:connections> </bns:EnvironmentExtensions>';


Above request string is the body that i am passing in the HTTPRequest.
i would appreciate your help!