Learn English – Is “pidgeon” a correct alternate spelling of “pigeon”?

We reference the Cambridge Advanced Learner's Dictionary.

In British English, we write television programme and computer program.

In American English, we write television program and computer program.

's shows either possession, or when the following word i.g. is/us/... is abbreviated. Therefore, in this case, 's can only be used if you're talking about something that belongs to "thank", which makes no sense at all; nor if you're saying "thank is" which does not make sense, either.

The s at the end of the word "thanks" is just a plural s and adding any apostrophe before that is just an error. Native speakers in each language may happen to make mistakes.

Word origin:

Date: 1300-1400 Origin: Plural of thank 'gratefulness' (11-17 centuries), from Old English thanc 'thought, gratefulness'

Says the subscription-only LDOCE.

To make your appreciation/gratefulness in a more formal way, you could say:

Thank you so much for your assistance.

Update:

As mentioned in the comment, if "thank" is being used as a verb, s would be added to make it third person singular:

She always thanks her mother for the meals she cooks.

However, there's another note that can be a common mistake among learners or non-native speakers who happen to use the verb form with that third person singular s for all persons; so instead of saying

"Thank God" or "Thank heavens" or "Thank goodness",

they'd say:

"Thanks God", etc.

Here, the full sentence would be: "I thank God for ...", and obviously, "I" is first person, rather than the third.