Either word (submittal or submitted) could work.
If you wrote:
proposal submitted by Sarawut Positwinyu
that would mean that Sarawut submitted the proposal. The proposal would the subject, submitted is the transitive verb, and Sarawut Positwinyu is the object. (Using submitted as a transitive verb here requires the proposal to be submitted by someone, or to someone, such as a committee, or for some reason.)
If you instead wrote:
submittal by Sarawut Positwinyu
submittal is a noun (meaning, that which has been submitted), and the prepositional phrase merely qualifies who submitted the proposal.
As for getting some guidance from the dictionary, check out this entry from Collins. You can see submittal listed as a noun, and submit listed as a verb. In your example, submit would be used as a verb under Meaning #3, to refer something for judgment or consideration.
As Roaring Fish indicated, submitted could also be used as an adjective, as in:
The committee reviewed the submitted report.