How can I get the all fields of webform created in Campaign through API?

I want these 2 fields Company Name & Domain from API.

I assume CompanyName is just the standard company name field and Domain is a custom field. All fo which you get via a standard retrieve contact using the relevant optional_properties https://developer.infusionsoft.com/docs/rest/#!/Contact/getContactUsingGET

You can get the allowed optional_properties via the model endpoint https://developer.infusionsoft.com/docs/rest/#!/Contact/retrieveContactModelUsingGET

How can I relate to campaigns and contacts then? For eg, I want contacts of the particular campaign ABC.
How to achieve this?

How can I link contacts with campaigns?

Can you be more specific on what you are trying to do? I don’t understand your question. Are you asking how can you add a specific contact to an existing campaign? If you achieve this various ways checkout the API calls under https://developer.infusionsoft.com/docs/rest/#!/Campaign. You can add a contact directly to a sequence, but you need the sequence id, which you can get by retrieving the campaign. You can also add them to the campaign by using
a campaign API Goal. https://developer.infusionsoft.com/docs/rest/#!/Campaign/createAchieveApiGoalEventUsingPOST

Actually I need the fields labels added in campaign webform.In my case, I have added two fields company name and domain.
So the requirement is to get those labels (company name and domain) after selecting the particular campaign.

I guess I am still unclear.

It seems like you have added a standard field Company and a custom field Domain (which are both Contact fields). When a Contact fills out the webform then these fields will be set on the Contact record.

If what you mean by Label is the actual text next to the field in the Web Form then no that text is not available via the API.

You’ll need to describe the use case and process. From where we sit, terms like “select the campaign” and others have no meaning so we can’t be sure of what you’re asking.