Select the channel that will receive data from the form created on Tilda. If such channel doesn't exist, click Create a new channel, then click Add Incoming WebHooks Integration.
Copy the WebHooks URL and switch to Tilda. On the same page below customise the integration by changing the channel, your user image and so on.
Go to Site Settings > Forms > Slack.
Insert your WebHooks URL, copied from Slack, into a corresponding field. If you have your own domain or you plan to connect one in the future, add all domains where you plan to place the forms, to Additional domains. Click Save.
If you did everything right, the page with a list of receivers, including Slack, will upload to your website.
Slack will receive a message from Tilda confirming your specified channel has been connected.
Return to the page with the form and open the menu in the Content block. Put a tick against Slack. Publish the page. The form is ready.
If something isn't working, please make sure your page opens at the same address as the one you specified in the form settings.