You can see different examples of having a button with a link in Email (will not allow to properly preview Button #4) or CloudPage and only Button #2 and #6 are not working, other variants should be sufficient for you to use:
%%[ SET @link = Lookup("DE_Name", "URL", "Field_identifier", "Value_identifier") ]%%
<body>
<p>Button #1 - Example of adding an onclick event to the button tag</p>
<form action="%%=v(@link)=%%" method="post">
<input type="submit" value="Button #1">
</form>
<br/>
<p>Button #2 - Example of adding an onclick event to the input tag</p>
<form>
<input type="button" onclick="%%=v(@link)=%%';" value="Button #2" />
</form>
<br/>
<p>Button #3 - Example of opening a link from a button in a new window</p>
<form action="%%=v(@link)=%%">
<button type="submit">Button #3</button>
</form>
<br/>
<p>Button #4 - Example of creating a button acting like a link with the formaction attribute</p>
<form>
<button type="submit" formaction="%%=v(@link)=%%">Button #4</button>
</form>
<br/>
<p>Button #5 - Example of opening a link from a button in a new window</p>
<form action="%%=v(@link)=%%" method="get" target="_blank">
<button type="submit">Button #5</button>
</form>
<br/>
<p>Button #6 - Example of adding an onclick event to the button tag</p>
<button onclick="%%=v(@link)=%%">Button #6</button>
</body>
The last, 7th variant also allows you to put %%=v(@link)=%% into the "Link URL" field inside Button properties when you are creating an email via Drag&Drop approach and once clicked open the link.