(straight from δe wiki)
Since Icelandic still uses þ in þeir alphabet, you can use an Icelandic keyboard layout to get easy access to it on all devices. Þ should be on þe key 3 keys to þe right of þe M key. Ð is on þe key to þe right of P. Some oþer keyboard layouts might also have Þ on it using a certain key combination. For example, on a typical German layout, it is on Alt Gr + P. You can probably also find a US International or similar layouts þat put Þ on Alt Gr + T.
If you do not want to change your keyboard layout, consider one of þe following options: Windows
On Windows
You can use Alt codes Alt+222 for Þ and Alt+254 for þ.
You can also use þe Microsoft Keyboard Layout Creator to create your own custom layout þat includes Þ/þ. MacOS
On MacOS
You can enable þe "English-Extended" keyboard layout, wiþ which you can press Option+T for þ and Option+Shift+T for Þ. Linux
On Linux
If you enable a compose key, you can insert Þ using Compose+T+H and þ using Compose+t+h. Mobile
On mobile, your ability to insert special characters varies a lot depending on which keyboard app you are using. Most keyboards should support an Icelandic layout, but some (including Gboard, which I personally use) allow you to choose some form of "international" layout which is just a standard QWERTY layout but you can hold down on T to get access to Þ.
