Short answer: Install the .ttf on your computer and restart Photoshop. For Hindi, Bangla, Tamil or Urdu, also switch the text engine to World-Ready Layout in Preferences, then Type, and start a new document.
1. Install the font
Download the font from the generator and extract the ZIP. On Windows, right-click the .ttf and choose Install (or Install for all users). On a Mac, double-click it and choose Install Font. Restart Photoshop and the font appears in the font list.
2. Fix broken Indic and Urdu letters
If your Hindi shows र्य as separate pieces, or Bangla conjuncts fall apart, Photoshop is using a text engine that doesn't shape those scripts. Turn on the World-Ready engine:
- Open Edit, Preferences, Type on Windows (Photoshop, Settings, Type on a Mac).
- Under Choose Text Engine Options, select World-Ready Layout. In older versions it's called Middle Eastern and South Asian.
- Click OK and create a new document. Text layers made from now on are shaped correctly.
Existing text layers keep their old composer. Retype them in a new document, or select the layer and choose the World-Ready composer from the Paragraph panel's menu.
The shortcut: use a PNG instead of the font
If Photoshop won't import the font, or Hindi, Bangla, Tamil or Urdu letters break apart in it, skip the font entirely. Type your text in the generator, pick a font and style, and press PNG. You get your text with its outline and shadow on a transparent background, already shaped correctly, which you can place on your thumbnail like any image.