Use a form to automatically replace placeholder text in preset prompt

I am developing an app which integrates with Chat GPT to help a specific target audience to create relevant marking content for their business. I would like to create pre-written prompt templates with [placeholders] which will automatically be replaced once they fill out a handful of form fields.

The way I envision it, they will fill out the form fields, and click the submit button, and the form fields will replace the placeholders in the pre-written prompt template and then send that updated prompt to Chat GPT. Does anyone know how I might accomplish this?

To be clear, this would be different than adding modifiers to the end of an existing prompt.