It sounds like you were on the right track with setting up the hint text. The built in hint text for text fields can’t be changed through interactions. However, because hint text style effect just turns the text gray and gets removed upon typing, we can create a custom version of this!
First, you’ll need to set the font color of the text field to gray. Then, add a focused style effect that has the font color to black like in the two screenshots below:
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Then on the dropdown, create the conditional logic for each of the options. You already had the start of this setup done but if you add more options it will look like below:
You can also set a max character limit to text fields by adding a “Text Changed” interaction like in this screenshot:
Then you can add conditional logic as desired if you only want this limit to apply to certain droplist options. You can take a look at the properties and expressions within Axure RP by navigating to our reference pages linked below:
I’ve made these edits to your file and reattached it for your convenience. Hope that helps and if you have any questions please let us know.