Rule Machine\Button Controller does not allow the use of variables for text fields

[bertabcd1234]

It actually does. Someone asked this question yesterday, and here is how you do it:

String Variable support for Actuator Parameters

You can already do this. Type %variableName% as the text, where variableName is the name of your variable. You can do this as either your entire text or as part of your text; the value will be substituted wherever %variableName% appears. This trick works in most places that accept string input, and the UI calls out many of these (e.g., on the "Send/speak a notification" screen) but apparently not this one.

tl;dr - you can just type %variable-name% directly in your text (no drop-down needed since, unlike the other inputs, this one already allows you to type in arbitrary text)