The CTA "Add post" is in itself a sentence. The word post is (I'm assuming) not a product name but rather a word like any other word in a sentence and should be treated as such. Using the Title Case should be limited to writing titles, for CTA's (Call To Action) (<-Look, Title Case), you should use Sentence case because that lowers the cognitive load of reading the text.
However, assume that this was a different scenario. And you wanted to know about how to format a label saying "my posts which linked the user to an overview of all her posts. This label is a Title and not a CTA (Since the label doesn't start with a verb). Here it's OK to use the Title Case because it's a practiced convention. Using a Title Case in mere sentences, which also happens to be a CTA labels, is violating the conventional formatting style.