Firstly I would like to thank Eliot Harper for creating this amazing site Ampscript Guide.
CreateSalesforceObject
To demonstrate I have created a Custom object called Animal in Sales Cloud.
With a standard field Name and a custom field Animal_type

Next you must create a Cloud-Page as below (change the UI as required).

%%[
if RequestParameter("submitted") == true then
set @createAnimal = CreateSalesforceObject(
"Animal__c", 2,
"Name", RequestParameter("AnimalName"),
"Animal_type__c", RequestParameter("AnimalType"),
)
endif
]%%
<!DOCTYPE html>
<html>
<body>
%%[ if not empty(@createAnimal) then ]%%
<p>%%=v(@createAnimal)=%% record created in Animal Object</p>
%%[ endif ]%%
<h2>Register</h2>
<form action="%%=RequestParameter('PAGEURL')=%%" method="post">
<label>Animal Name: </label>
<input type="text" name="AnimalName">
<label for="AnimalType">Animal Type: </label>
<select name="AnimalType">
<option value="Pet">Pet</option>
<option value="Wild">Wild</option>
</select>
<input name="submitted" type="hidden" value="true" />
<input type="submit" value="Submit">
</form>
</body>
</html>
Then once you submit the form,

A new record will be created in Sales cloud and 18digit record ID will be returned.

You can use this ID to find the newly created record in Sales cloud.
