You will need the Full SMS permission to perform these actions. You will also need the Contributions Settings permission to perform these actions.
First, you will need to set a keyword for the designation, then you will need the steps to set up the text message contents.
Setting a keyword for a designation
From the left-hand navigation menu, go to Contributions > Settings, and select "Edit" from the dropdown at the end of the designation you are trying to modify.
Under the Options section of the pop up locate "Text Giving" and select "Add Text Giving Keyword."
You need to create a Keyword for this designation, as shown in the screenshot below:
Be sure to "Save" your changes.
Using "Text to Give" to donate to a specific designation
"Text to GIVE" donations can be specific to a designation. After you have created a keyword under the designation as described above, your donors can now add the keyword after the word GIVE and the amount they intend on donating.
GIVE + AMOUNT OF DONATION ($25) + KEYWORD (TEST)
Example of an SMS a donor would send: GIVE 25 TEST