insert from popup

[kusalo]

I have worked out how to insert from the popup

<a href="javascript:window.opener.document.FORMNAME.FIELDNAME.value='set here';window.close();">"link"</a>

But this overwrites all existing.
How do I add my new value to existing content?

I am short on knowledge and the above string is more accident than anything else so please feel free to suggest alternatives.

I am building a 'insert special character' table - all the math bits and greek and such.
I have tried working with smiley and calendar equivalents but can't seem to pull out the necessary.

thanks

[kusalo]

got it sorted

Use the "+=" operator instead of the plain assignment ("=") operator.