With this product-specific custom field you enter the link text (title) for the corresponding link. It is displayed as the label of the link on the product page.
Each of the three links (Further link 1–3) has its own title field.
You can find the "Further link 1 – Title" custom field in the respective product:
Tab "Specifications" ⇒ Block "Custom fields" ⇒ Tab (Set) "Further links (... Pro)"
Type: Text
Technical name: twt_{theme}_{edition}_custom_field__product__further_link_1_title