Help: Form Submission to Salesforce, No SF Experience

Please forgive me. I've never dealt with Salesforce before. I'm working on a small side gig project coming into some pretty tough code that I've been muddling through to the best of my ability. It's been a shit week, and I'm exhausted. But the people I'm working with are wanting a conversion from where a form is currently being submitted to Salesforce somehow by tomorrow. I've been waiting over a week to get access to the Salesforce account, not that I have any idea what I'm doing anyway, and I still don't have that. I've set up a developer account to see if I can figure anything out about processing this form and I'm so out of my depth.

I've managed to set up a connected app and find the right settings to get a token back when using a username and password, but I know that won't fly and isn't the right thing to do within my code. The current setup has an endpoint URL, the path to add the form submissions which is appended to the URL, and an API key used for authorization.

I have no idea if the client knows any of this stuff. I tried to ask the people I'm working with for an endpoint URL yesterday and that turned into them saying they'll request my own login to Salesforce. Is there anyone in this group that might be willing to talk to me and help me figure out what I need to try to get my developed setup working so I can figure out how to submit a form and add the info to I guess the Contact list? I don't even know if that's what they want. All I know is the login screen for the client's Salesforce account says marketing cloud.

Is there a way to get an authentication key that is permanent the way the current solution's API key is set up? I feel like theoretically I should be able to swap out some values and probably update the JSON for the form contents to make this work but I really don't know, and I'm not wanting to have to learn Salesforce tbh. If this isn't going to work, I guess I need help figuring out how to authenticate with every form submission?

Also, are the field inputs for adding a contact the same for all Salesforce environments or is that something that the client could potentially change? All we're dealing with is first name, last name, and email address. They have other form fields like country but I guess they aren't planning to store that anymore.

If I can figure out what to do with the developer setup, in theory I'll just need to change things out to match the client's account to process the intake of I ever get that info.

Please, friendly Salesforce people, talk me off the ledge.