Hi Elly and Philipp,
I came up with another idea to send thank you email after each form submission.
We can create a flow and add "When a HTTP request is received" action, which is very similar to HTTP request trigger in Azure logic app,
however, compared with Azure logic app, the benefit is that no extra subscription plan is required to do it in flow directly.
Please take overview:
The action will create a custom API endpoint for us, we can call it with our request data and get our data in output.
What we need to do is building a sample JSON schema to receive data and generate output.
e.g:
{
"type": "object",
"properties": {
"message": {
"type": "string"
},
"mailbox": {
"type": "string"
}
}
}

I sent a simple ajax post request to generated endpoint.
var body = {
"message": "Iam clofly",
"mailbox": "123@mao.com"
}
$.ajax({
url: 'https://xxx.logic.azure.com:443/workflows/981f3c6ca56c441eb3b8f2dec460f118/triggers/manual/paths/invoke?api-version=2016-06-01&sp=/triggers/manual/run&sv=1.0&sig=8xCE292mKJ_dzfQ86egs86uv-H8jBCZ9AD46Dt3v33Y',
type: 'post',
dataType: 'json',
contentType: 'application/json',
data: JSON.stringify(body),
success: function( data ){
console.log(data);
},
error: function( errorThrown ){
console.log( errorThrown );
}
});
Result:
We can get property data easily from output:

So the solution would be:
1. Create such flow to send email to contact.
2. Call the endpoint at form onSubmit event. (See the tutorial https://docs.microsoft.com/en-us/dynamics365/marketing/developer/marketing-form-client-side-extensibility)
It's easy to get input fields data and assign them to custom property.
I don't test it, but it shall works.
In addition, we can add a custom field to form to save validation data and send it with other submission information,
then check the value with condition action, which could help us prevent malicious URL request.
Regards,
Clofly