Hi @Who
There are a few ways of achieving this depending on how you are planning on implementing your solution and how much control you have over the form page where the submission is coming from.
Option 1 is to make use of a Custom Connector.
Custom Connector overview can be found here
https://docs.microsoft.com/en-us/connectors/custom-connectors/
Details on how to implement a Webhook trigger in your Custom Connector can be found here
https://docs.microsoft.com/en-us/connectors/custom-connectors/create-webhook-trigger
Option 2 is to use the HTTP Request Trigger
Whilst this isn't necessarily a webhook as such, it does allow you to very easily generate a url that you can pass data to (I tend to prefer a JSON object in the body of the request) and then process within the Flow.
Details on this trigger can be found here
https://docs.microsoft.com/en-us/azure/connectors/connectors-native-reqres
Your URL will be automatically generated when the Flow is first saved. When defining the body data I find its a lot easier to create a structured example like the below first, and then pasting this in as a sample response to generate the schema (see screenshot).
{
"variableA":"valueA",
"variableB":true,
"variableC":200
}

All of your values within your JSON body will then be available as dynamic content.

Hopefully this helps you get started.