Apply via Switzerland: Liechtenstein is in Schengen but has no visa office. Swiss embassies handle Liechtenstein visas. In Bangkok, you apply as if you are applying to Switzerland (through TLScontact). On your application, list Liechtenstein as main destination, but know that the Swiss Embassy will issue the visa.
Documents = Swiss requirements: Use the Swiss visa checklist. Essentially, to visit a partner in Liechtenstein, prepare the same docs as visiting Switzerland. This means a solid invitation letter from your partner in Liechtenstein, their proof of residence (since Liechtenstein residents often have Swiss permits or similar), and possibly the financial guarantee procedure (Switzerland’s) if needed.
Swiss guarantee process: Be aware that the Swiss (hence Liechtenstein) authorities might trigger a guarantor process. If your finances are limited, the Swiss cantonal authority might ask your partner to sign a déclaration de garantie (letter of guarantee) at their local office. This can happen shortly after you apply – your partner might get an email or letter to come in and provide documents. It’s routine; just ensure your partner follows through promptly.
Return ties: Like Swiss visas, showing strong ties to Thailand is important. Even though Liechtenstein is tiny, the scrutiny is the same. Demonstrate you have a job or family obligations to return to. A well-written cover letter from you, addressed to the Swiss Embassy, explaining you’ll visit Liechtenstein and return to Thailand, can personalize the case and help.