Create a single tag, then create a post linked to that tag. Uses async/await.

post_with_tag.js

const remote_domain = 'DOMAIN', // without trailing slash
			application_password = 'APPLICATION PASSWORD',
			username = 'USERNAME';

		const headers = new Headers({
			'Content-Type': 'application/json',
			'Authorization': 'Basic ' + btoa(username + ':' + application_password)
		});

		const makePost = async function() {

			const tagResp = await fetch(`${remote_domain}/wp-json/wp/v2/tags`, {
				cache: 'no-cache',
				mode: 'cors',
				credentials: 'include',
				method: 'POST',
				headers: headers,
				body: JSON.stringify({
					'name': 'fudi'
				})
			});

			const tag = tagResp.json();

			let tags = [];

			if (!!tag.id) {
				tags.push(tag.id);
			} else if (!!tag.code && tag.code === 'term_exists') {
				tags.push(tag.data.term_id);
			}

			const post_data = {
				title: 'Post using REST API',
				content: 'Post content using REST API',
				status: 'publish',
			};

			if (!!tags.length) {
				post_data.tags = tags;
			}

			const options = {
				cache: 'no-cache',
				mode: 'cors',
				credentials: 'include',
				method: 'POST',
				headers: headers,
				body: JSON.stringify(post_data)
			};

			fetch(`${remote_domain}/wp-json/wp/v2/posts`, options)
				.then(data => data.json())
				.then(data => console.log(data.link))
				.catch(data => console.error(data));

		};

		makePost();